I got the following tables:

extension
======
id
address
forward_no_answer
forward_busy
forward_all
status

user_extension
==========
id
extension_id
back_code
back_date

user
===
id
user_extension_id
first_name
last_name
password
status

How do I define that user_extension is a subclass of extension?
I've checked the docs and I'm not really sure on how to configure it, only that I need to add "subclass" tag.

*bump* =)