You don't have to enter your own part #'s, they are optional. However, if you don't enter a part #, then the parent part # will show, so you'll have to go by the product name to determine which product to ship.

ie.

Part #1000 - T-Shirt Color: Green Size: M

and if they ordered Size L and color Yellow, part # would still be 1000. The inventory is taken care of correctly in 3dcart's side, but the part # will be the same on invoice, etc. So if you need the part # to be a particular number, then you need to enter them, if not, you can leave them blank.