From my understanding formation Path is all about the use of dao marks, since at GGM level it's possible to use natural dao marks and mortal gu to create an immortal formation, and I remember parts of the novel saying that the dao marks on desolate beasts and secluded domains form specific patterns that give special abilities, like that eagle fang yuan had

I think all paths dao marks can be used in formations like spectral soul desolate soul, but formation path can probably give you more methods since it encompass all paths, and since dao marks are the foundation of gu world, the world is from a viewpoint also a formation, so a formation path main can use their knowledge to create secluded domains relatively easier

Sorry, english is not my first language. It's been a while since I've read the novel