Canon 5D Mark III User Manual (en)

 File Numbering Methods
The file numbering restarts from 0001 each time the card is 
replaced or a new folder is created.
When you replace the card, create a folder, or switch the target card 
(such as 
g), the file numbering continues in sequence from 0001 
for the images saved. This is convenient if you want to organize images 
according to cards or folders.
If the replacement card or existing folder already contains images 
recorded previously, the file numbering of the new images may continue 
from the file numbering of the existing images in the card or folder. If 
you want to save images with the file numbering starting from 0001, use 
a newly formatted card each time.
To reset the file numbering to 0001 or to start from file number 
0001 in a new folder.
When you reset the file numbering manually, a new folder is created 
automatically and the file numbering of images saved to that folder 
starts from 0001.
This is convenient if you want to use different folders for the images 
taken yesterday and the ones taken today, for example. After the 
manual reset, the file numbering returns to continuous or auto reset.  
(There will be no Manual reset confirmation screen.)
Auto Reset
Manual Reset
Card-A (f)
Card-B (g)
File numbering after 
replacing the card
File numbering is reset
File numbering after 
creating a folder
If the file number in folder 999 reaches 9999, shooting will not be possible 
even if the card still has storage capacity. The LCD monitor will display a 
message telling you to replace the card. Replace it with a new card.