A special 'meta-tag' is applied to each committed change to the repository, which uniquely identifies that commit. This is randomly generated string, and has no significance except that it is unique. It is used to group files from a single commit into a changeset.

To operate on a single the command the @ and @< prefixes define the committed revision, and the revision before the commit respectively.

For example:

$ cvs diff -r "@<91c41475ddc4ad2" -r @91c41475ddc4ad2 foo.c
Index: a.txt
===================================================================
RCS file: d:/repo/test/foo.c,v
retrieving revision 1.2
retrieving revision 1.3
diff -r1.2 -r1.3
...

Note that many shells treat the < symbol as special, so that part of the command will need to be quoted.