Original: On modern host adapters (since about 1997), doing I\/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let the operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID.
Simplified: Transferring data on the SCSI bus is managed by the host adapter.