Useragent and sdpsession

I have several users on my asterisk-1.6.2.x, i.e. joe, jane, etc. I wonder if both useragent and sdpsession are global or peer variables. The reason I ask is I want to use these variables on a peer basis so that when each account registers to its respective VoSP server, they will identify itself as its username on Asterisk PBX system, instead of the default values.