It is a bit weird and took me a while to understand. Crates are non ordered as has been described, so in my workflow I import my Traktor Playlists for types of music as crates so for example I have a master Drum & Bass crate with sub crates for different styles and moods. This is where I will flick between when playing live and on the fly.
I import “set” playlists from Traktor as playlists, and these will retain a numerical order. I quite like keeping them separate so if I want to play from or find an ordered set then I flick over to playlist mode.
It means that I might start a set playing from a pre-arranged playlist but then decide to go off piste and click onto crates and go digging through, but if I click the playlist button then I’ll be right back where I was on my arranged set. So in essence there’s hardly any difference at all. If syncing between Engine Prime and Trakor or others perhaps it would be good to have a field to define whether each playlist from Traktor is to be designated as a crate or playlist in EP. Maybe default to crate unless you specify playlist?