The Rental field is used to identify Inventory items that are rented rather than sold.
The Rental Sales Transaction Type should be used for the initial transaction. When entering rental line items, each should be entered on a separate line, even if the Customer rents more than one of the same item. The Quantity for each line item represents units of rental time (i.e., hour, day, week, etc.).
When an item is marked as a Rental item, it is rented out instead of sold. The Quantity On Hand is reduced and the Auaocated Quantity is increased. When the Customer returns the item, the Rental Transaction is converted to a Sales Invoice, the Allocated Quantity is reduced, and put back into Quantity On Hand so it may be rented out again.
See:
Inventory Database - Main Screen Fields