Spinner

Something is happening and there is no shape to hold. The second choice after a skeleton — and the name is required.

When to use it

A button is submitting. Button has loading and draws its own — this is for the places where there is no button.
A panel waiting on an answer whose size nobody can predict. A skeleton would hold room for something other than what arrives.

The shape of what is coming is known. Then IngotSkeleton says more and, above all, holds the room — so nothing jumps when the data lands.
The wait is longer than a few seconds. After half a minute a spinning circle no longer says "working" but "stuck" — a wait like that needs progress or a sentence.
A whole page loading. A blank page with a circle in the middle is worse than a page that shows its shape and fills in.

label: string — What is happening, already translated ("Saving"). Required: a spinning circle with no name is, to a screen reader, a picture of nothing.
size: "sm" | "md" — Defaults to `sm`. Both match the height of a `Button` of the same size, so a spinner beside a button sits on one line.
block: boolean — Centred on its own line — for a panel that is waiting. A button keeps the inline shape.
className: string — Placement only — margins, alignment.
testId: string — An anchor for tests.