$node->buy_add in ec_roles_save

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.

Obviously I can't just leave it as buy_add in my module, as it would clash with ec_roles' provided variable. I need to figure out how to load in my own version of that variable.

Thanks in advance.