Canon EOS 1D Software Instruction Manual page 171

Click this checkbox when you want to save the image without changing the name.
When the image files to be saved include
images with sound, this option can save
the sound files with the respective images.
This is text inserted before the file number. You can enter up to 12 characters.
When you enter text, an example appears in the [Example] field below the text box.
Click this checkbox when saving the file after converting a RAW image to a
JPEG image or TIFF image adds one of the following at the end of the file name.
Conversion Format
JPEG
TIFF (8bit/ch)
TIFF (16bit/ch, 16bit/ch linear)
Alternatively, you can perform the same operation using the menu that appears by
holding down the <control> key and clicking.
Enter the number of digits in the sequential number
that attaches to the file and the starting number
used. A maximum of 9 digits can be specified.
File Name
RJ
RT8
RT16, RT16L
Saving Images
M
3
M-47