In most cases, an isolated and embedded view provide all the layouts a Card needs. However, a developer may want to offer many different layouts based on the viewport for their Card, given the variety of screen sizes and places someone might use a Card. We call these "variants."
For example, suppose you are building a calendar widget. You might want it to have different styling based on its container size. If someone is using the calendar in a very small form, you could use the small size. If it is in a larger view, such as a calendar of events, it could show more detail. And then finally, it would have a stand-alone, isolated view.
This is a future feature of the Card SDK.