"branch" field in the admin section of RCS files... What's it for?

[Russell Yanofsky]

( This is a technical question, not a bug report. The page at
http://www.cvshome.org/communication.html seemed to indicate that the
bug-cvs list was appropriate for this sort of thing, but the archives are
another story. Anyway... )

I am writing a small application which outputs a CVS repository. I thought I
had grokked the RCS file format, and I even have most of the application
written, but just now I was looking over some CVS files and I cannot figure
out what is the purpose of the branch field at the top.

The rcsfile manpage says:

admin ::=  head {num};
      { branch {num}; }
      access {id}*;
.
.
.

which means the branch field is optional and can hold at most a single
revision number. But if a the file has multiple branches, which branch
number should be inserted there? What does it mean if one branch number is
there as opposed to another one? Why is the field even neccessary at all?

Thanks for your help.

Russ Yanofsky