[VoIP] Congestion vs Hangup in Asterisk

[Greg Blakely]

Hi, folks.  
 
I'm looking for a distinction between CONGESTION and HANGUP, and when
each should be used.
 
CONGESTION is defined as "send congestion notification and wait for
hangup,"  while HANGUP is pretty simple: "Disconnect the call."
 
Okay, so I know that I can use HANGUP everywhere, and things will work
smoothly.  But I'm wondering where CONGESTION would be more appropriate.

 
And, it has to be asked, is CONGESTION sent towards the originating end?
If so, would it apply only to incoming calls?  Incoming from where? a
PSTN line?  an extension?
 
 
My head hurts!