Suppose you have two related products to be released together. Alternatively, suppose to have a project and some separately developed associated components. In either case, when you tag the released version, it would be helpful to tag them atomically in the same commit. If this is done, a view of the history of the creation of such a tag will include the associated tags too.
This can be done with svnmucc (multiple URL command client), included with the SVN command-line client distribution.
Thus, rather than doing this:
svn copy -m"Tagged version 1.0" -r100 http://repos/trunk/project1 http://repos/tags/project1/v_1.0 svn copy -m"Tagged version 1.0" -r123 http://repos/trunk/project2 http://repos/tags/project2/v_1.0
Instead, you can do this:
svnmucc -m"Tagged version 1.0" -U http://repos/ cp 100 trunk/project1 tags/project1/v_1.0 cp 123 trunk/project2 tags/project2/v_1.0
[Each of these commands is supposed to be on one line.]
which is probably the best source of information at the moment. I have not yet found a good reference elsewhere on the Internet; svnmucc is discussed on the SVN mailing list, but there’s little about usage. [There is also the source.]