Normally, you should define milestones using the "By date" criteria. This method is simple, quick and powerful. For example, you may need a revision based on the date of a ZIP file or CD delivered to a customer. Then, you may define a milestone based on this date.
The "RCS Symbolic Name" criterion is mainly used by organizations that share the CS-RCS repository with Unix repository or manipulate the RCS files by scripts. This method is performed in two steps: first, the desired revision of each file is marked with an RCS symbolic name. Then, you can define a CS-RCS milestone based on the given RCS symbolic name.