"Fall Sub Only" is not a standalone league where you will be placed onto a team to play. In MUFA, players are required to be registered for a specific season to participate in that season.
The Fall Sub Only option provides an option for those who are not registered in one of the current Fall Leagues (such as FMP Sun, T/Th Mixed, etc.) to be able to sub in this Fall season, if the team needs a sub.
How to Find Sub Opportunities
If a team needs a substitute, they typically post a request in one of these sub groups: