The Epicor 9 Fulfillment Workbench option is only available if you have licensed the Advanced Materials management module. It allows a finer grain of control when fulfilling orders (whether sales, build-to-order, plant transfer, etc).
One of the most confusing things is the difference beween allocating and reserving. Both options associate a part and quanity with a given order, and make that quantity unavailable to other orders. Both can be undone, making the quantity available again.
The difference between reserving and allocating is the granularity of control. When you reserver a part for an order, you can reserve that part across all warehouses, or just the primary warehouse. When you allocate a part, you can allocate a part from a specific bin or lot, or even by serial number.