Shouldn't exp be the expansion slot to add more input slots like additional cartridge slots, hard disk slots and so on?

It works when I simply remove the exp parameter:
mame64 c64p -bios r3 -ramsize 65536 -iec10 "" -iec11 "" -iec4 "" -iec8 c1541 -iec9 "" -joy1 "" -joy2 joy -tape c1530 -user "" -cassette "" -floppydisk "" -cartridge batman -quickload ""

But then, what value should I set for exp so that it works?