The Svn plugin is useful if you would like to track changes to your bcfg2 repository using a Subversion backend. It deprecates the previous Subversion integration mentioned here at ftp://ftp.mcs.anl.gov/pub/bcfg/papers/directing-change-with-bcfg2.pdf.
Currently, It enables you to get revision information out of your repository for reporting purposes. Once the plugin is enabled, every time a client checks in, it will include the current repository revision in the reports/statistics.
Future plans are to commit changes to the repo which are made by the server (adding clients, ssh keys, etc).
Simply add Svn to your plugins line in /etc/bcfg2.conf:
[server]
plugins = Base,Bundler,Cfg,..,Svn