Custom Functions menu description F/W E3kr111B71 (16-06-2004)
CF 01:

SET button func. when shooting

Works: Yes
Default: 0:Default (no function)
Preferred: 2:Change parameters
Specifies how the SET button functions:
0: Makes choices when you highlight commands on the menu.
1: Changes image size and quality when you hold down SET and turn the Quick Control Dial.
2: Toggles parameters setting when you press SET, then turn the Quick Control Dial to adjust parameters.
3: Displays the menu.
4: Switches to playback mode.
What is FEC?
BigMike from DPReview's Forum explains:
Easy to install, and works as advertized. The FEC is a really nice function to have. For new users that don't know what FEC is, it stands for Flash Exposure Compensation. In short, it allows you to adjust the strength of the flash higher, or lower. This is very helpful for many shots. The FEC can be turned on or off in the menu. You can have your "set" button work the FEC, or have it
bring up the normal "quality setting" menu. Nothing is disabled, everything works fine. Just thought I would mention it for those who were interested.
Many 300D users have been waiting for this feature.
CF 02:

Shutter release w/o CF card

Works: Yes
Default: 0:Possible without CF card
Preferred: 0:Possible without CF card
Specifies if the shutter works when there is no CF card in the camera.
CF 03:

Flash Sync speed in Av mode

Works: Yes
Default: 0:Auto
Preferred: 1:1/200sec. (fixed)
Sets the shutter speed to automatic, or fixed at 1/200 when using flash in Av mode
Setting it to fixed prevents a slow shutter speed when photographing in dim light. Set to 1/200, no more walking inside at weddings and getting half second exposures! Could actually be useful as it lets you force 1/200s with flash in Av instead of automatic slow synch.
CF 04:

Shutter button / AE lock button

Works: No
Default: 0:AF/AE lock
Preferred: N/A
Has four settings that control how the shutter and AE-FE Lock button marked with the asterisk (*) work. These combinations determine how and when focus (AF) and exposure (AE) are locked:
0: Pressing the shutter button halfway down locks exposure and focus. Pressing AE-FE Lock locks exposure.
1: AE lock/AF—Pressing the shutter button halfway down locks exposure. Pressing AE-FE Lock locks focus.
2: AE/AF lock, no AE lock—In AI Servo AF mode, pressing AE-FE Lock temporarily locks focus when an object passes in front of the camera so focus isn’t affected. Exposure is set when the exposure is made.
3: AE/AF, no AE lock—In AI Servo AF mode, pressing AE-FE Lock starts or stops focusing. Exposure is set when the exposure is made. This option is useful when photographing a subject that repeatedly stops and starts.
This is probably one of the most wished for settings, but it didn't seem to work.
CF 05:

AF-Assist Beam / Flash firing

Works: No
Default: 0:Emits/Fires
Preferred: N/A
Specifies how the flash’s AF assist beam and flash fired during the exposure work together:
0: Enables both the AF-assist beam and the flash.
1: Disables the AF-assist beam and enables the flash.
2: Enables the AF-assist beam on external Speedlite and enables the flash.
3: Enables the AF-assist beam and disables the flash.
This one does not appear to work.
When choosing from the menu, there are two settings separated by a slash. The AF assist beam is listed first by whether it emits or not, and the flash is listed by whether it fires or not.
CF 06:

Exposure level increments:

Works: Yes
Default: 1:1/3-stop
Preferred: 1:1/3-stop
Seems to work. At least the steps between aperture change on the
display. Remains to be seen if it has consequences on the hardware.
Seems to be about exposure compensation. Seems to work, but the LCD on the back doesn't seem to have the right amount of "ticks" to display this correctly.
CF 07:

AF point registration

Works: No
Default: 0: Center point
Preferred: N/A
By registering the AF point you often use, you can switch to it instantly. You can register an offcenter AF point which suits your preferred framing or automatic selection for the shooting moment priority. Any of the seven AF points can be registered. Only one AF point can be registered.
CF 08:

RAW+JPEG rec.

Works: Yes
Default: 3:RAW+Medium Fine
Preferred: 0:RAW+Small Normal
The jpeg is embedded on the RAW. You can use Canon's FVU to extract it.
CF 09:

Bracket. sequence/Auto cancel.

Works: No
Default: 0:0,-,+/Enable
Preferred: N/A
Has four settings that specify the order in which the three images are captured when using exposure or white balance bracketing. When making your selection, the choices are given with three settings separated by commas. 0 stands for the camera’s suggested exposure/white balance, - is underexposure/bluer, and + is overexposure/redder. The command also specifies if bracketing is enabled (left on) or disabled (turned off) after a series has been finished.
CF 10:

Superimposed display

Works: No
Default: 0:On
Preferred: N/A
This is to have the focus points (red leds in the viewfinder) either to turn off after focus is locked or to stay on.
It is not to have more displayed informations.
CF 11:

Menu button display position

Works: No
Default: 0:Previous(top if powered off)
Preferred: N/A
Has three settings that determine which menu item is highlighted when you press the MENU button. The first choice returns you to the most recently selected command or to the top menu choice if you have turned the camera off since last using the menu. The other two choices return you to the most recently used menu choice or to the top menu choice. With it at "previous", when you visit a menu in which you had previously set the selector to some item down the list, the selector will initially be there when you next visit it.
CF 12:

Mirror lockup

Works: Yes
Default: 0:Disable
Preferred: N/A
When enabled, you can lock the mirror up so it doesn’t add any vibrations when you take a photo. This is a very useful feature when taking macro close-ups, or using very long lenses. When on, pressing the shutter button all the way down raises the mirror and pressing it all again fires the shutter and lowers the mirror.
If you turn on both mirror lockup and the self-timer, pressing the shutter button all the way down raises the mirror and then fires the shutter some seconds later. This delay time can be set at 1.5, 3, 4.5, 6, 7.5, or 10 Sec. using CF.17, in this new firmware release. (When you release the shutter button you’ll hear a shutter-like sound, but it isn’t the shutter.) If you don’t have a remote control, this is a great way to avoid camera shake from pressing the shutter button.
WARNING: When using mirror lockup, don’t point the camera at the sun with the shutter button pressed halfway down. Doing so can damage the shutter curtains.
NOTE: Your camera must be set in single shot mode, not in continuous mode.
TIP: You may leave MLU enabled in the CF menu, and switch with the drive mode button between continuous drive mode and single-shot mode, in order to select or deselect MLU. This is a lot quicker!
CF 13:

Assist button function

Works: No
Default: 0:Normal
Preferred: N/A
Doesn't seem to have any function, since the 300D AFAIK doesn't even have the assist button.
But another button may eventually be defined for this later.
CF 14:

Auto reduction of fill flash

Works: No
Default: 0:Enable
Preferred: N/A
Disabling it prevents the underexposure of a subject against a bright background.
CF 15:

Shutter curtain sync

Works: No
Default: 0:1st-curtain sync
Preferred: N/A
0: Fires the flash as soon as the shutter is fully open.
1: Fires the flash just before the second curtain closes.
Possibly tonight's greatest disappointment. It didn't work. I even tried it with an external manual flash. Still doesn't work. It has been tried with a 420EX too, without success so far.
Doesn't seem to do anything, at least with built-in flash. Tried a 2sec exposure in M mode, but flash fired at the beginning. AF Lock didn't help either. Tried the 2nd curtain but it doesn't work, presumably the hardware doesn't support it.
CF 16:

Safety shift in Av or Tv

Works: No
Default: 0: Disable
Preferred: N/A
When enabled, exposure corrections will be made at the last possible moment if lighting changes. When enabled, the following can happen:
In the shutter-priority (Tv) mode, if a correct exposure cannot be obtained with any aperture set by the camera, a faster or slower shutter speed is used.
In aperture-priority (Av) mode, if a correct exposure cannot be obtained with any shutter speed set by the camera, a larger or smaller aperture is used.
CF 17:

There are two versions of E3kr111B71 firmware.
The oldest one shows CF 17 as follows:

Mirror lockup pause time

In the newest B71 version, spelling in MLU menu's for many languages has been corrected.
Also in the English language setting, CF 17 spelling has been slightly changed (particular for this one, a suggestion of me in the Yahoo 300D hackers forum). A newer version has become available under the same name, it is now also for download local on this site. It shows the English verion of CF 17 as follows:

Mirror lockup pause time

Works: Yes
Default: 0:1.5 Sec
Preferred: Any choice
MLU (Mirror Lock Up) prevents mirror-caused vibrations which may blur the image during close-ups or when a super telephoto lens is used. It can be enabled in the Custom Functions menu, CF 12. This function controls the time that expires between mirror lock up and actual exposure. When you press the shutter Button fully, you can hear the mirror flapping up. The camera vibrates at this time because of the mass of the mirror that slapped up. After for example 3 seconds, the vibration is gone, the shutter is released and a picture will be taken that is (much) more sharp.
Old CF 17:

Lens AF stop button function

Works: No?
Default: 0: AF stop
Preferred: ?
Has settings that control focus, exposure, and image stabilization on super telephoto lenses with AF stop buttons.
The 300D doesn't have a lens af stop button.
Now, this one looks the most promising of all, but I don't know what it truly means, and I seriously doubt it will work based on previous functions.