Bushwalk grades

Is there an official way to note the difficulty (easy/medium/hard) of a bushwalk? The Australian tagging guidelines only mentions that highway=footway is the right tag. It would be nice if it could be put in the name or ref tags, as these both display on my GPS.

perhaps a custom POI with this detail in the description or perhaps there is a generic tag for this type of user data?, perhaps a post in the general section might get some help.