I learned this from owning a Nikon D500 and trying to figure out why the checksums of files on the SD Card and XQD card don’t match.
On a D500, which has two card slots, an XQD and an SD, this data is recorded in the file under the exif field ‘Memory Card Number’.
0 is the value for the XQD Card slot.
1 is the value for the SD Card slot.
relevant stackoverflow link