New "notched" implication feature on copyright and characters? What is it/how to use it?

Check out the copyright section on this post #11453376.

It has this new feature which is pretty new to me, where the copyright has a "displayed implication" on it.
So in this case it's

idolmaster 275k
->idolmaster cinderella girls

This is also the case for characters with multiple designs, like on Kantai Collection kai ni designs. What is this feature called? And how is it used, both on danbooru, and also on forked instances of danbooru, if admin access is required?

"Nested" is the proper term for this. It was added in December 2023 (here).

They automatically display copyright, character and meta-tags like this if one implies the other, see help:tag implications for more info.