Reference is RelicBox's own number for the item and is what goes on the labels you print. Leave it blank and the next number is used. Product barcode is only for a barcode the item already came with, such as the EAN on a boxed game, so scanning the box finds it. Most items won't have one, so leave it empty.
If it's inside another item (e.g. a card inside a computer), it takes that item's location automatically.
Change the list of descriptors in Settings.
RelicBox