The reference on “composite Baselines” is: “Best practices for using composite baselines in UCM”.
Introduced in ClearCase v2002, composite baselines are a mechanism for grouping baselines into a collection. One baseline is designated as the composite, and other baselines become members of the composite
You do not necessarily have to use composite baselines when you have multiple components, but:
If you want to remember the set of baselines existing at a given point on your Stream, then having a rootless component able to record of the baseline of its member is useful.
In theory, you can have “composites of composites”:
I wouldn’t recommended it, though, because of the “parasite baselines (explained briefly here and here)