Apparently there are so many superheroes in the future that some of them get jobs as substitutes. Many heroes in the future can join the Legion of Super-Heroes, as long as they finish college with a degree in Super Powers, pass student-superhero-ing, and score high enough on the accredited tests. After they qualify for and pay for their superhero license, they can apply for a job with the Legion of Super-Heroes. If the economy is bad and the Legion isn’t hiring due to budget cuts, the heroes can join the Legion of Substitute Heroes. They get called up if a regular superhero is sick or is gone for professional leave.
These heroes also go to the Legion of Substitute Heroes if they aren’t quite qualified to be in the regular Legion. Perhaps they earned their degrees, but they didn’t score high enough on the accredited tests. Or, perhaps they tried a career in another field, but thought they’d want to try out superhero-ing. Regardless, these are the second-string heroes it seems more like a carnival sideshow than a super club.
Additionally, if there are so many people with powers in the future, why do the superheroes need to band together? Who do they need to protect? Does the Legion just spend their time protecting the Substitutes?