A Segment allow you to centrally manage content intended to be broadcast across multiple channels. When you add a segment to a channel, you have the option to unlink the segment.
While a segment is linked, any changes made to the original will apply any time the segment is being used in a channel. The segment's lineup cannot be changed from within channel details. If you wish you make edits to your Segments, you must go back into the Segment tab to make those changes.
Unlinking a Segment permanently dissociates it from the original segment. In effect, it creates a copy of your segment in the channel. This will allow you to make changes to the Segment details in the Channel configuration without affecting the original segment that might be used in other channels.