They are the employee and supplier discounts.
A - Active employees
Z - Retired employees
X - Suppliers
A little bit more information here - http://www.fordemployeeplan.com/pricing.html
The nice thing, in addition to the varying discount, is that it is no negotiation. The price is the price.
We were able to use it when we ordered a 09 Mariner Hybrid (6 month wait), so that makes me hopeful they will honor them with the FFE too.