Yeah, exactly. Different paints can call for different products. Back in the day #7 was the *perfect* product for automotive paint, but times have changed and today's less porous paints don't let #7 penetrate; it just sits/floats on the surface and only a little bit of it gets down in the "furrows" of any marring. Gotta watch you don't inadvertently clean too much of it out/off when applying subsequent products- you *can* put products over it, but you gotta remember what you're working with. The "pure polishes" that dry more completely (than #7) just make more sense IMO for basecoat/clear paint. Not that people don't get good results with #7+NXT (I'm sure not knocking people who like this combo), but I still reach for other products for this sort of application even though I have #7 on the shelf.
Seems like the appearance and ease-of-use are inversely proportional; #7 looks best but is trickiest to use, #5 looks the "worst" (relatively speaking) but is the easiest to use. #3 and #81 are in between the two. FWIW I like #5.