The input to the SCSI driver is a generic IOCtl request packet, pointed to by the ES:BX register pair.
Note: It is not the intention of the IOCTL definitions in the SCSI.SYS specification to provide an application level programming interface to SCSI devices. The intention of the IOCTL definitions is to allow client drivers at their Ring 3 INIT time to communicate with the OS2SCSI.DMD which is running at a higher privilege level than the installable driver.
A client driver may choose to provide application level services. However, all responsibility for locking and removing LDT references from the IOCTL/SCB/TSB structures would rest with the client driver not OS2SCSI.DMD.