Sip.conf register extension question

i have a usage question to the sip register command (only for incoming calls)

According to the asterisk documentation i should use
register user:password@host/extension

I have two accounts, so i set up:
register userid1:password@host/1
register userid2:password@host/2

This works fine, but i cannot see the calls in the call log of my sip provider.

They told me to use
register sipid:password@host/sipid

so i changed to
register userid1:password@host/userid1
register userid2:password@host/userid2

and renamed 1 and 2 within extension.conf

This works fine and i can see the calls in the call log of my sip provider.

But now i have two questions:
why is the extension transmitted to the provider? does this parameter has two functions?

if i have two sip accounts with the same userid (different providers), how can i distinguish them in extension.conf?

register 0815:password@provider1/0815
register 0815:password@provider2/0815