Final Cut Pro X: storylines
All instances of the timeline contain a primary storyline, which is the main sequence of clips that you build to create your movie.
Storylines are sequences of clips connected to the primary storyline. You can use storylines for the same purposes as connected clips (such as creating cutaways, compositing titles and other graphics, and adding sound effects and music).
Storylines allow you to edit a sequence of connected clips within the context of the other clips in the timeline. For example, you can add cross dissolve transitions to a series of superimposed titles in a storyline and then adjust the timing of the titles to match clips in the primary storyline. Like connected clips, storylines can contain both video and audio, or they can be video only or audio only.