For example, you are sellings eggs. In your warehouse, you manage the eggs individually. Lots of eggs are scrapped and you do it egg by egg. The unit of measure is Unit(s).
If you are selling eggs by the dozen, the selling unit of measure is the Dozen. You will set it on your sale order.
The cardboard trays that contains the dozen of eggs is the packaging.
When you are selling several trays, you might wrap all the trays into a box or in a plastic wrapping. It is the package.