When used, this parameter will display a field at build-time so that the user is able to select a Subversion tag from which to create the working copy for this project.
Once the two fields Name and Repository URL are set, you must (1) ensure the job uses Subversion and (2) set the Repository URL field of Subversion by concatenating the two fields of this parameter. For instance, if Name is set to SVN_TAG and Repository URL is set to https://svn.jenkins-ci.org/tags, then Subversion's Repository URL must be set to https://svn.jenkins-ci.org/tags/$SVN_TAG.
Notice that you can set the Repository URL field to a Subversion repository root rather than just pointing to a tags dir (ie, you can set it to https://svn.jenkins-ci.org rather than https://svn.jenkins-ci.org/tags). In that case, if this repository root contains the trunk, branches and tags folders, then the dropdown will allow the user to pick the trunk, or a branch, or a tag.