This works in conjunction with the "This project builds a Subversion feature branch" checkbox above. This build step merges any revisions in the upstream branch, via svn merge, and commits them. This is normally called rebasing in Subversion. Automatic rebasing is useful to keep branches close to the upstream, thereby reducing the risk of integration problem down the road.

You can rebase from the latest revision in the upstream branch, but you can also do safer rebase by choosing the permalink. For example, if you specify "last stable", then Jenkins will look for the last stable build, then rebase with the corresponding Subversion revision. Thus you know that you are rebasing to the known good state.

If you don't want Jenkins to automatically rebase, you can still use the "Rebase From Upstream" link from the left to manually initiate the rebase.