Inventory and Parts List are used interchangeably throughout the program and the help.
A part can be added to an RO without it being in inventory. The program knows this and does not try to reduce the inventory, where an item that is added via the inventory or Parts list is recognized by the software and reduces the inventory item accordingly.
The huge advantage of adding items to inventory (whether you are tracking the item or not), is you will be able to select that item the next time you use it. If you doubt you will ever use the item, you can simply add it directly to the Repair Order.