I am going to quote
@iaresee from about 3 years ago (i searched for this too)
"Yes, it means "Global". If you enable global for that parameter than the value used comes from the Setup > Global Settings > Config page under the EFFECT MIXING section. There are two parameters there, Reverb Mix and Effects Mix, that let you change the value for any block using the global section, globally, from that one place.
It's a nice way to say, dial back
all your reverbs in your presets if you're in a room that's really boomy, without having to visit every REV block in every preset you're going to use and do it there.
https://wiki.fractalaudio.com/wiki/index.php?title=Reverb_block#Rev_Mix_.28Global_Menu.29"