What Scheduled is
Where an email you've asked to send later waits until its moment comes.
Scheduled sits below My work in the sidebar and holds exactly one thing: Send Later, every email you've asked TeamEmail to send at a time you chose rather than the moment you finish writing it.

An email lands here the moment you choose a send time instead of sending straight away, from the arrow beside Send in the composer. See Send Later for finding, editing or cancelling one.