Track View provides two special track types: Note and Visibility. Note tracks let you annotate keys with information such as what the key is doing or how you plan to edit it, and Visibility tracks let you change an object's visiblity during an animation.
With Add Note Track, you can insert a note track below a highlighted track in the Track View Hierarchy. You can make note keys and associated notes at specific time locations to keep track of what keys do. Do this byin Dope Sheet mode, then right-click a note key to see the Notes dialog. The information you enter appears in the Key window under the keys.
Use Remove Note Track to delete a note track and its associated keys.
Add Visibility Track controls when you can see an object. Visibility tracks can only be added to objects.
Use Remove Visibility Track to delete a visibility track and its associated keys.