Birds & Bees: Where do "Page Settings" come from?

Heya!

Still getting around WP. On my main site, when editing a PAGE, I have something called "Page Settings", where among other things, I can add custom BODY CLASS for that page.

I need this for pages on my other site (multisite), but not sure where it comes from. Is this coming from: 1) native WP feature 2) ThemeCo X Theme Function (active theme on main site, not second) 3) Other?

Is there a way to add "Body Class" or Body ID field to the Page Edit forms somehow?

Tx!

Ira