We have a 5-segment GL structure as follows: Entity-GL Account-Cost Center-Department-Reporting Unit. We use a linked list to connect our Entity-CC-Department so our AP team cannot pick an invalid string. Now 3-years post implementation, we are wishing we had included the Account Number (Expense Type in Concur) in the linked list. We have been told recently by Concur that the system field "Expense Type" would have had to be in the first place of thelinkedlist. And, once the linked list is set up, the system field cannot be added into the first place of the list. So, we are stuck with what we have unless we want to rebuild our linked list using all new custom fields. Just something to keep in mind when you are building your linked list.
To answer your question about invoice implementation - yes. Invoice uses the same linked list as expense.