Card

The surface content sits on — not a border drawn around it. It can lift on hover, and once per screen it can invert to dark.

When to use it

The content belongs together and should be visually separated from the rest of the screen — a settings panel, a summary, a tile in an overview.
You need a header with a title: CardHeader and CardTitle come with it and keep the same padding.
A tile that leads somewhere — hover lifts it on pointer-over. The card itself is not clickable: wrap it in an <a> or a <button>.
A platform message that has to out-shout its surroundings — tone="dark". It inverts the surface, so a second one next to it out-shouts nothing.

The card would contain a single other card. Nested surfaces tell the reader nothing extra, and the padding compounds until the content is lost in the middle.
You only want padding. That is a className on a wrapper; a card also draws a border and a background nobody asked for.

elevation: "flat" | "raised" — How far the surface stands out. Defaults to flat.
hover: boolean — Lifts the card on hover. Only for clickable tiles — and that tile has to be a link or a button.
tone: "default" | "dark" — An inverted surface for a platform message. At most one per screen.
padded: boolean — Turn off when the content owns its padding (a table filling the card).
className: string — Pass-through class — for layout, not for recolouring.