When creating a new slot on the your station's Schedule, you have the option of selecting a Show, a Playlist, a Pilot or a Relay Stream option (Show being available only on the Professional plan).
In case you have scheduled more than one items on the same time slot (e.g. 14:00-17:00), then there is a priority order. In other words, your scheduled items come in effect following a certain order of precedence, only in the case that the item of top priority fails to air. The idea behind the ordering goes from the most specific to the most generic, i.e. Show → Playlist → Pilot when starting from highest to lowest priority.
Let's see an example. Say you've scheduled at the same time slot a Show and a Playlist. If you fail to log in on Radiojar to do your Show (and assuming it has no content), then the Playlist will start. If the Playlist finishes airing before the slot is over, then, as always, the station's default Pilot will kick in.
Keep in mind that schedule items of different time slots should not overlap each other (e.g. Show 01:00-04:00 & Pilot 02:00-03:00), as this can cause any of them to not air correctly.