2+++ means that it still is a classic envelope design but one side is in 30 mm= 1 3/16" mesh and the other in 40 mm = 1 5/8" mesh.

You are free to choose which side you hang towards the wall and from which side your horse is supposed to eat. The smaller mesh is for softer hay, like hay large, and the 40 mm = 1 5/8" mesh is for more coarse hay or straw.
Why choose, when you can have them both?