PRCS, the Project Revision Control System, is the front end to a set of tools that (like CVS) provide a way to deal with sets of files and directories as an entity, preserving coherent versions of the entire set.
PRCS was better in some aspects than CVS or other VC software of the day as it can manage several files in a tree as a set and assign a single version for them. But it is no longer recommended for new projects as there are better tools for distributed collaboration today.
PRCS is a useful tool. The only drawbacks are, the project itself is abandoned, and PRCS only compiles with gcc-2.