Refill guarantee
A promise to top an order back up if the delivered engagement disappears — which is an admission that it is expected to disappear.
A refill guarantee says: if the followers we sent you drop off within some window — 30 days, 60, sometimes 'lifetime' — we will send more.
Read that carefully, because it is the most honest thing on most panels' websites. A refill guarantee is a commercial admission that the product decays. Nobody guarantees to refill something that stays.
The guarantee is also only as good as the panel's willingness to honour it. It is a promise from a reseller whose own supplier may vanish, and it is not an escrow, a warranty, or a contract you can enforce anywhere.