This determines whether 1 or 2 partitions will be created. If the card is 2GB or less only a single partition can be created. If the card is bigger than 4GB then two partitions must be created - a small 16MB one containing DISKBOOT.BIN and the other will hold the CHDK/SDM folders (SCRIPTS etc), photos and movies. By default, a 4GB card can be formatted as either 1 or two partitions. You can override the default by editing the file.