This is from the discussion started here:
https://hudson.dev.java.net/servlets/ReadMsg?list=users&msgNo=770

The issue is that specifying a CVS tag in the branch field results in a cvs co command that has a tag and
a date. A cvs client will checkout all of the files and then delete them. The fix would be to allow the user
to specify either a CVS branch or a tag on the configuration screen and then issue the checkout
command without the -D date option if a tag was enabled.

This requirement comes from the use of CVS tags to specify a certain mix of files from the repository.
(e.g. the next TEST build) The tags are moved from revision to revision to select what goes into the next
build. To my knowledge, this cannot be implemented as a CVS branch. Initially I thought that this was
an example of non-standard CVS use however I have done some surveying of other team leaders I know
and at least one other group of developers at a different company have the same process/requirement.
It appears there was also a long discussion of CVS tagging on the SVN list a few years ago where people
also indicated a similar usage of CVS: http://svn.haxx.se/dev/archive-2004-04/0131.shtml