It’s an address, just the same as the 2000 in SIP/2000 is an address. It is the address you use to access the agent in the Dial application, e.g. for agent 001, you use:
exten => 1234,1,Dial(Agent/001)
just as you would use:
exten => 1234,1,Dial(SIP/567890abcd12)
Note that making the SIP address the same as the extension that accesses it is bad practice because:
it makes it easy for an attacker to guess good user names when they try to register;
it makes you think that there is a one to one relationship between Asterisk extensions and SIP devices, when there is no such restriction.