EC Roles

Hi Everybody, I'm currently hacking up the code of EC Roles to handle paid subscription to taxonomy terms.

I'm trying to re-create ec_roles_save(), and I'm wondering if somebody could advise me on the line:
$buy_add = serialize(array_intersect_assoc($roles, $node->buy_add));

Specifically-- where does $node get loaded up with buy_add? I can't seem to figure it out from browsing through the rest of the source for the module. Is it something obscure going on in the form logic? It doesn't look like it ever explicitly creates a 'buy_add' attribute to latch onto node.