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 35 mm = 1 3/8" 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 35 mm = 1 3/8" mesh is standard SlowFeeding mesh that work well for almost everything.
Why choose, when you can have them both?