If a company resells serial numbered products, it is often the case that the serial numbers are already printed on or attached to the products as barcodes when the company receives them from its supplier. Thus it is common to use the serial number barcodes for internal stock record keeping also, rather than assigning new serial numbers.
The time at which you typically record these serial number into your inventory records is when you receive the products into your inventory as a shipment. As you unload boxes from the truck or unpack items from a box, you scan in the serial number barcodes and associate them with the items in the inventory system that they represent.
In some situations it is impossible to print out specific barcode labels for products in advance. If a supplier commonly makes substitutions, for example, then you can't know in advance of receiving a shipment from that supplier exactly what it contains. Therefore you can't know exactly what specific barcode labels to print to stick onto the boxes or items as you are receiving them. In situations like this, a useful approach for barcode identification is to assign barcodes on-the-fly.
The scenario is this: a truck arrives at your facility with a shipment of items. To receive these items, you want to unload the boxes from the truck and apply barcode labels to them for your inventory management software. But what labels to apply?
To assign barcodes on-the-fly, you simply print out a booklet of generic barcode labels. Each barcode represents a unique number, but it doesn't matter what the number is. They could just be sequential, 1,2,3,4, etc., but make sure they don't overlap with barcodes that you've used before.
Then as you unload a box from the truck, you pull off one of the generic barcode labels and stick it on the box. Select the product ID that it represents and scan the label. From this point on, Finale remembers the associations between the barcodes that you put on the boxes, so if you later scan these items to do a transfer or sale from your inventory, the scan key correctly identifies what the items are.