A Spotify account has a list of devices. A device is selected as one that will actually make sound.

When you log in to the same account on more than one device the others become remotes for the one that’s curently playing.

That’s just a great piece of design in my opinion.

Of course the one device limit is in their interest but out of all the ways that it could be enforced this is just really clever.

Not only is playback not interrupted, and no warnings or errors are needed but the functioanlity is actually useful. It’s also very easy to understand.