I’m not sure what you’re asking, but here’s a stab at an answer. When you nest a sequence (dragging the sequence from the Browser into the timeline of another sequence), FCP makes a single clip of that entire sequence that functions as any ordinary quicktime clip in the timeline. If you hold COMMAND while dropping the sequence you want to nest into the timeline, FCP will recreate the nested sequence, track for track (respectively, depending on where you place the nest), in the timeline – it’s basically like copying and pasting and entire sequence from one timeline to another, just a lot faster and more fun. I hope that helps.
Best,
Ben