Apple Final Cut Pro X (10.0.9) - Manual (Page 105)

Chapter 8 Edit your project 105
The first frame of the source selection is connected to the clip in the primary storyline at the
playhead or skimmer position. If you dragged the selection to the Timeline, the first frame of
the source selection is connected to the primary storyline at the point where you released the
mouse button.
Connection point
Connected clip
Primary storyline
When you rearrange, move, ripple, or remove clips in the primary storyline, any clips connected
to them will move (or be deleted) along with the primary storyline clips.