outlet group ID open channel flow
 

Hi everybody.
I am simulating a open channel flow.
Anyone can explain me what is Inlet/outlet Group ID? With some practical examples referred to a open channel flow for instance (air+water).
I read tutorials (Fluent tutorial) but the description is too "contorted" and difficult, with some sentences that say opposite things!
http://aerojet.engr.ucdavis.edu/flue...ug/node936.htm

Thank you