In AX the GL account selection when you post documents usually comes from a posting profile. The posting profiles can work on an table which is a specific association, group based on the group the record belongs to or All records. In the case of sales posting the table could refer to item or customer, group would refer to item group or customer group. This is a quick walk through of setting up a new sales account to have sales post through from orders for the customer group 10.

Cheers

Lachlan