Customer Parts lets you enter part numbers for a customer to automate their purchase.
If they always buy the 4-pack of your item, ABC, they might already be using ABC-4 as their part number. Or they may use the same, ABC, but purchase Unit Count, 4.
When new item, ABC+, comes along, assign their old code, ABC, to get them the new. You can switch them to a new or substitue item without altering their purchasing.