doIntercept call semantics

[Polar Humenn]

Inside an interceptor, I'll assume the "handleMessage(Message message)" 
call, what is the purpose of calling InterceptorChain.doIntercept(message)?

Is it right to assume that after a call to doIntercept() the chain is 
completely executed?

So is code placed after this call provide some clean up or end fixing to 
the message?

Also, can somebody tell me the intended semantics of 
InterceptorChain.doInterceptInSubChain() and a definitive use-case?

Thanks,
-Polar