What is the Callback transport?

I hope my question is not dumb:
( but I'm not familiar with the callback transport)
 
1) Is that a common Linux transport interface?( Is is a local machine
interprocess interface?)
2) Is it net-snmp transport?
3) Do sub-agents by default (like when using the example template), use
the callback transport?
4) How is interprocess communication achieved with the callback
transport?
5) Where can I read about? (not easly found in google)
 
Thanks,
Erez Makavy