Many modern device controllers use the SCSI command set as a protocol to communicate with their devices through many different types of physical connections.
In SCSI language a bus capable of carrying SCSI commands is called a "transport", and a controller connecting to such a bus is called a "host bus adapter" (HBA).
The file drivers/scsi/scsi_debug.c simulates a host adapter with a variable number of disks (or disk like devices) attached, sharing a common amount of RAM. Does a lot of checking to make sure that we are not getting blocks mixed up, and panics the kernel if anything out of the ordinary is seen.
To be more realistic, the simulated devices have the transport attributes of SAS disks.
For documentation see http://sg.danny.cz/sg/sdebug26.html