The SCSI mid level is common to all usage of the SCSI subsystem. Probably its most important roll is to define internal interfaces and services that are used by all other SCSI drivers. These internal mechanisms are not discussed in this document [see ref: W2].
The primary kernel configuration parameter "CONFIG_SCSI" determines whether the mid level is built in (when "=y") or a module (when "=m"). If "CONFIG_SCSI=m" then all other SCSI subsystem drivers must also be modules.
When the mid level is built as a module then it probably never needs to be loaded explicitly because using 'modprobe' to load any other SCSI subsystem module will cause the mid level to be loaded first (if it is not already).