Final Cut Pro X: compound clip
You can use compound clips to group any combination of clips in the timeline or the browser, and nest clips within other clips.
Compound clips can contain video and audio clip components, clips, and other compound clips. Effectively, each compound clip can be considered a mini project, with its own distinct project settings. Compound clips function just like other clips: You can add them to your project, trim them, retime them, and add effects and transitions.
When you add media (clips) to the contents of a standard clip, the clip automatically becomes a compound clip.
Compound clips have many uses. You can:
Simplify a complicated project by creating a separate compound clip for each major section
Sync a video clip with one or more audio clips and then combine the clips into a compound clip, to avoid inadvertently moving them out of sync
Open any clip, edit its contents in the timeline, and then close it
Quickly create a compound clip containing the clips in an event, based on the browser sort order
Use a compound clip to create a section of a project with settings different from those of the main project