The host adapter drivers support all levels of the SCSI standard: SCSI-1, the Common Command Set (CCS, superceded by SCSI-2), and SCSI-2. Not all optional features of the standard are supported. Different systems support different feature combinations (such as synchronous, fast, and wide SCSI), depending on the available hardware.
The host adapter drivers handle the low-level communication over the SCSI interface, such as programming the SCSI interface chip or board, negotiating synchronous or wide mode, and handling disconnect/reconnect.
A host adapter driver is not, strictly speaking, a proper device driver because it does not support all the entry points documented in Chapter 15, "SCSI Device Drivers." You can think of it as a specialized library module for SCSI-bus management or as a device driver, whichever you prefer. The software interface to the host adapter driver is documented under "Host Adapter Facilities".
Caution: Connect/disconnect strategy is enabled on any SCSI bus by default (the option is controlled by a constant defined in the host adapter driver descriptive file in /var/sysgen/master.d). When disconnect is enabled on a bus, and a device on that bus refuses to disconnect, it can cause timeouts on other devices.