Although you can have multiple mainline streams in a stream depot, we recommend that you create only one mainline for each stream depot.
Having a single mainline stream in a stream depot provides a neater organization and better visual representation in the Perforce Visual Client (P4V). It also avoids the likelihood of re-parenting a child stream into the "wrong" mainline stream hierarchy by mistake.
The Perforce Server allows any stream to be re-parented to any other stream as long as both have the same depot root. With only one mainline per depot, you can be assured that re-parenting will never result in a stream whose parent belongs to a different mainline from which it was originally branched.
However, in some cases you may want to maintain multiple mainlines. For instance, maintaining customized streams for several customers may require several de-facto mainlines as described in Chapter 6 of the Perforce Directory Standard.
We know that our existing customers have been using Perforce successfully without streams. If the added power and flexibility of our streams implementation appears useful and fits your development environment, try it out! Otherwise, you can continue to use Perforce as before. If you have any questions, please contact Technical Support.