nusergroupid and pusergroupid

[Mko]

How are these variables defined, as I am trying to make it so upon a Payment being activated, a user's primary usergroup is changed in another database, and upon a Payment being completed, the user's primary usergroup is reverted back to the Registered Users usergroup.

The way I'm using them:
nusergroupid = $sub['nusergroupid']
pusergroupid = $user['pusergroupid']

Code (Paidsub_build):

$query = "UPDATE characters SET mgroup = {$sub['nusergroupid']} WHERE id = {$userid}"; 

Code (Paidsub_delete):

$query = "UPDATE characters SET mgroup = {$user['pusergroupid']} WHERE id = {$userid}"; 

Thanks for any help.