Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 266)

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z [Auto reset]: When you wish to restart the file numbering 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 useful if you want
to organize images by 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 on the card or in the
folder. If you want to save images with the file numbering starting from
0001, use a newly formatted card each time.
0051 0001
(1)
File numbering after
replacing the card
100
0051
101
0001
File numbering after
creating a folder
Card-2 Card-1 Card-1
(1) File numbering is reset