The copyright symbol.
Copyright striking is a feature on YouTube that allows people to claim copyrighted content on someone's videos which belong to them or other people for the YouTube staff to delete them.
On Mapperdonia, some anti-mappers, mostly T-Series Mapping and S2N, falsely used the system to take down entire mapping channels to destroy Mapperdonia, However, this didn't ended up well, The striking forced some channels to privatize all their videos, but no known mapping video was deleted because of the striking.