Hey Orion….I believe what it means is that the tags are valid for all land in the unit that is NOT national forest or "off national forest". From your description it sounds like you can hunt most of the unit that is the most productive antelope habitat.
This same type of description used for deer unit 105 (Region F). There is a season for "on national forest" and a different season for "off national forest".
Off national forest means all land not on national forest.