Each time you add a track to the playlist, a checkmark will be added on that line (up to a maximum of 3 checkmarks are shown per track), and a checkmark is also placed on the corresponding volume's line (so you know that one or more of its tracks are in the playlist).
You also can add an entire volume's tracks to the playlist, in a single operation, when in the Volumes view. Simply highlight the volume you want, then select Add to Playlist from the right-click menu.
To remove a track from the playlist, do any of the following...
Checkmarks are removed accordingly.
Note that a track or volume cannot be deleted from the library if it's currently included in the playlist. This limitation also applies to track modifications.