What Drops are
Every AI generation or edit produces usage, which the backend converts into whole Drops at the current rate. Drops are integers; there are no fractions.
Where Drops come from
Buying a membership grants purchased Drops plus bonus Drops, along with monthly and per-period quotas. Drops from different sources live in separate batches, each with its own expiry.
The spending order
Spending always takes from the batch expiring soonest, so short-lived Drops get used first. The billing view shows each deduction and the balance afterwards.
What reservations are
Before the AI starts working, it reserves an estimated amount of Drops so the job does not run dry mid-way. When the job finishes, the actual usage is settled and the extra comes back; cancel the job and the whole reservation is released.
That is why "available" can be lower than "balance". The difference is what is currently reserved.
Expiry rules
When a batch expires, its unused Drops are gone for good. They do not come back and are not reissued. The wallet shows the next batch about to expire; spend those first.