Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 205

[Auto reset]: The file numbering restarts from 0001 each time
the card is replaced or a new folder is created.
When you replace the card or create a folder, the file numbering
restarts from 0001 for the new images saved. This is convenient if
you want to organize images according to cards or folders.
However, 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.
File numbering after
replacing the card
Card-1
XXX-0051
File numbering is reset
[Manual reset]: To reset the file numbering to 0001 manually 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.)
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.
For both JPEG and RAW images, the file name will start with "IMG_". Movie
file names will start with "MVI_". The extension will be ".JPG" for JPEG
images, ".CR2" for RAW images, and ". MOV" for movies.
Card-2
100-0001
Handy Features
File numbering after
creating a folder
Card-1
100 101
XXX-0051 XXX-0001
205