Strange available and purgeable space on High Sierra
Oddly enough I had just under 60GB of purgable space.

I read around and found a few posts where people mentioned creating large files as once you ran out of space osx would start using purgable space. It works but you don't need to!

You can run bootcamp assistant- it will remove local time machine backups AND icloud cached content. Not only did i get my 60GB purgable space back, but i got an additional 60GB.

Considering I'm on a 250GB SSD and want to dual boot, extremely pleased. Hope this helps!