What are the exact criteria for this tag? Seems to me that Exhibitionism covers most things that would be referred to as "public indecency".

...Or is it a joke tag that flew over my head?

I just read one chapter with this tag, but I'm gonna assume it means committing acts of exhibitionism without seeking the thrill of it. You're not doing it for sexual reasons, you just do it because it doesn't bother or embarrass you if people notice. That's just a guess though