This value is used as the template of svn commit message, and populated as the defualt when this plugin is enabled. You can embed the following variables with groovy string format.
env
Map returned by hudson.model.AbstractBuild.getEnvVar().
sys
Properties returned by System.getProperties().
For example, you can embed BUILD_TAG in the comment.
        Tagged by svn-tag plugin. Build: ${env['BUILD_TAG']}