Canon EOS REBEL T8i Advanced User's Manual page 640

Continuous
When you wish to continue the file numbering sequence even
after the card is replaced or a new folder is created.
Even after you replace the card or create a new folder, the file numbering continues in
sequence up to 9999. This is useful when you want to save images numbered anywhere
between 0001 to 9999 on multiple cards or in multiple folders into one folder on a computer.
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 use continuous file numbering, it is
recommended that you use a newly-formatted card each time.
File numbering after replacing the card
Card 1
Card 2
(1) Next sequential file number
File numbering after creating a folder
Card 1
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