What happens when you try to remove a booster pack feature quantity that is already in use?

If you try to remove a booster pack feature quantity that is already in use, the system allows you to remove only the unused quantity. It also displays the following error message, Cannot reduce the quantity, please remove a few records and then try.
For example, for One Enterprise edition, if you add quantity - 5 for a custom module booster pack, a total of 30 (25+5) custom modules will be allowed.  If 29 out of 30 custom modules are being used and you try to remove quantity - 3 i.e., enter the new quantity as 3, the system displays the following error message, Cannot reduce the quantity, please remove a few records and then try. And allows you to remove only quantity 1 (unused quantity).