CMOS is not journalism, so it's not variation from the GP?

A wider number of people use either of them. Every place I’ve used used CMOS which I now use with others.

Company I used to work for used AP for things like press releases and, I think, official blog posts and Chicago plus a couple different tech style guides for everything else.

Basically, we didn’t like some things in AP but we wanted to make it easy for journalists to copy/paste.