The threaded version has holes to attach side and bottom rails, the non-threaded does not.
For .308 versions specifically, the threaded versions has threaded holes on the sides to attach their Pic rails to. The non-threaded handguard has M-Lok slots.
The threaded version (308L) is also meant for low-profile 308 uppers. The M-Lok version (308H) is meant for high-profile 308 uppers.