jenkins/war/resources/help/_cvs/branch.html

<div>
  If set, Hudson will run CVS with <tt>-r</tt> to build a particular branch.
  If left empty, Hudson will build the trunk.

  <p>
  Normally, Hudson runs cvs check-out and update with the <tt>-D date</tt> option
  to make sure it gets a coherent snapshot of the repository and better build
  reproducibility, but this interferes if the branch name specified here is actually
  a tag name. Telling Hudson that the name is a tag would fix this problem.
</div>