The ROM cartridge contains DISK-BASIC 2.0 which is an upgrade to DISK-BASIC v1 which is integrated into any MSX with an internal disk drive, or to the ROM that is part of the disk interface cartridge for external drives. The exception to this are the MSX Turbo R systems which came with DISK-BASIC v2.
DISK-BASIC 2.0 is a pre-requisite for MSX-DOS 2.2x which is on the disk.