Backorders involve out-of-stock items that need to be ordered for the customer. Unless you are running an updated version of RMS, backorders require a 100% or 0% deposit depending on the setting under Manager | File | Configurations | Options.
Layaway accounts are committed inventory (or items that need to be ordered) which will be paid for over time. You can only make payments on a Layaway.
Workorders are for commited inventory for later pickup. This may or may not involve service or assembly. You cannot take payments on Workorders, but you can take deposits (Shift-F9 Details).
If your customer wants to put a deposit down and pay off the balance when the item is picked up, use a Workorder. Press Shift-F9 Details to take the deposit.
You can simulate a payment on a Workorder by increasing the deposit. Press F11 Recall, press Shift-F9 Detail, select Over-ride deposit, and enter a higher amount. The difference will be logged as an additional deposit.
If your customer wants to make payments on a in-stock or out-of-stock item, place the item on a Layaway and take a deposit. When the customer returns to make a payment, recall the layaway (F11) and use Shift-F4 to make a payment.
Special order items can fall into any of the three categories. You may need to special order a product a customer wants to make payments on (use Layaway), or place a deposit on an item that will be assembled and picked up later (use Workorder), or pay-in-full for a item you are currently out of (use Backorder).
For further information on this subject, including ordering items that don't currently exist in the item file, search this knowledge base for 'special' or 'deposit'.
Note: Quotes do not reserve inventory (committed stock). Quotes can be converted to workorders for processing (commit, order, receive, pickup).