Timeline Preferences

Defaults

Show Groups: Displays the Group layers in the Timeline.
Show Effects: Displays the Effect layers in the Timeline.
Show Sound: Displays the Sound layers in the Timeline.

Paste/Drag & Drop

Add keyframes at beginning and end: Select this option to copy and paste a set of frames that includes an interpolated sequence between two keyframes, but only extracts a partial section. Under normal circumstances, cutting and pasting a function without either the start or end keyframe would render the function null. This option caps the partial function with a new keyframe when you perform the paste.

Related Topics 

Timing Preferences
Exposure Sheet Preferences
Advanced Preferences