[Standards] Username Escaping with Gateway Registration

[Ian Paterson]

XEP-0100 "Gateway Interaction" doesn't, AFAICT, explain whether the 
username supplied at registration should be the "Legacy Service 
username" or the "Jabber username". [The difference between these 
usernames (typically escaping) is explained in section 6.2 (User 
Addressing).]

Can anyone please enlighten me on which username should be used?
I think it would be very useful to specify this in the XEP.

- Ian