Welcome :)
It is an abstract concept that, integrating with forum definition, is basically advanced for people who don't want to call themselves advanced. Basically, apply the standards of the advanced forum as per site definition as the minimum for a 'High Casual' game.
I also strongly suspect that it is literally advanced posted in casual to be more 'hey, people, come on in' friendly.
And assuming the title is a valid question, Guild Mods = collective account for staff team use to post things such as rules.