If "Create partitions" is ticked and two partitions are to be created, this will be the name of the second, main partition on the card. It will hold the SDM/CHDK folders as well as photos and movies (i.e. everything except DISKBOOT.BIN).
If "Create partitions" is not ticked, this field will be greyed out.
If only one partition is to be created this field will not appear.