Castle says that is just there to keep the switch in the housing. If you wanted to remove the switch from the housing in order to replace or troubleshoot it then youd straighten that tab which would allow the switch to slide out of the housing.
I'll have to put a meter on it. The esc has no way to mount a fan. It's a micro esc encased in some sort of super thick heatshrink
I'll shoot an email to castle too and ask them what it is