Unit: Synonymous with a Unit Directory. A unit has a one to one correspondence
with a device driver. From SBP-2: A component of a Serial Bus node that provides
processing, memory, I/O or some other functionality. Once the node is
initialized, the unit provides a CSR interface that is typically accessed by
device driver software at an initiator. A node may have multiple units, which
normally operate independently of each other. Within this standard, a unit is
equivalent to a target.
Service: A sub component of a unit that operates on a set of tasks independent
of other services of the same unit. For SBP-2, there is a one-to-one
correspondence between a service and a logical unit (LUN). For example, a
printer (unit) might implement a PDL service responsible for data flow to the
printer and a management service responsible for control and status.
Queue: A collection of ORBS that proceed or block independent of other queues.
by Atsushi Nakamura
My shot at definition of "function":
A function is part of the unit architecture and is an instance of a device
model, e.g., mass storage, CD-ROM or printer. A function is usually a
collection of services that are user identifiable.
The definition of a function seems to indicate that a function is a unit.