[Users] question about adding users

[B Boc]

I am now trying to "add users" but i don't  understand why username requires 
domain name and the implication down the road.

if i issue the command:
openserctl add 2002 spa3k user2 at mysipserver.com

I get the response:
"domain unknown: use usernames with domain or set default domain in 
SIP_DOMAIN"


why does it expect my username to be 2002 at mysipserver.com?

does this therefore mean the UAs will  have to have their username field set 
to username at domain instead of just username.  What's the relevance of 
capturing the email  address?