[AGENT++] Access to Management Information

[Haw, Stephen-P56341]

Perkins "Understanding SNMP MIBs" section 5.2.5 states:

"Each managed sub-system must include for the agent an access
mechanism to management instrumentation contained in the sub-system.
The instrumentation is the "extra" hardware or software that must be added
to the sub-system to measure and retain the value or act on a changed
value of management information."

What is this access mechanism and how does one determine it exist 
on the managed device and if not how is it implemented?