The spend cap is the most Quavsit can bill you for overage in one month. It is €20 by default, and you can set it anywhere from 0 to €1 000 on my.quavern.com/account/quavsit.
What it covers, and what it does not
It applies to overage only — the units beyond what your plan includes, and pay-as-you-go units. It does not cover the subscription itself, which is billed as usual.
When the cap is reached
The API stops serving and says the spend cap was reached. Calls resume:
at the start of the next UTC month, or
as soon as you raise the cap.
Departure boards and widgets stop with everything else: a refused charge puts a board into its unavailable state, and the screen says so. Nothing is served for free once the cap is hit.
It only matters if overage is on
Overage is off by default. With it off, a plan stops at its included units and nothing can be billed beyond the subscription. Turning overage on is what makes the cap the thing standing between you and an unexpected invoice.
Choosing a figure
Work out what you would accept to pay in a bad month, not what you expect in a normal one. Bad months are the ones this is for: a loop in a client, a widget on a page that suddenly does well, a board left running at a short interval. The per-widget daily cap is a second, narrower guard for that last case.
If you were billed more than you expected
In this order: whether overage is on, the cap on the account page, the usage per key for that period, and whether a board or a widget was running.
Quavern help reference: quavsit-spend-cap/en