The Tween menu is available when a media cue is selected. It allows you to add tween tracks to the cue. Likewise, tween tracks can be removed by unchecking the corresponding item on the Tween menu.
NOTE: The items available on the Tween menu vary depending on the selected media cue. For instance, Volume tweening is only available for sounds and videos – not for still images.
Read more about the various kinds of tween tracks under “Tween Tracks”.