[galaxy-dev] Meaning of ${on_string} in tool XML files

Hi all, What is the official mean/use of ${on_string} in Galaxy tool XML files? This variable name appears in lots of tools, but does not seem to be documented here: https://bitbucket.org/galaxy/galaxy-central/wiki/ToolConfigSyntax Are there any related variables which also need documenting? I've checked the mailing list (at least, for as long as I've been subscribed), and done a quick search with Google, but found nothing informative. Thanks, Peter