An exclusion is way to fine tune the pop up offer.

For example if you have an upsell:

Silver Drone but the client already ordered Gold Drone you can set Gold Drone as the Exception as you do not want to upsell Silver when they have Gold already.

When the Exception is met (IE Gold Drone ordered) the pop up offer will be skipped and your second choice will be shown.

In the Screen shot above - Drone Photography is the Pop up offer, it will be offered as long as Drone Video has not already been ordered.