When N2 is bigger than 1 the antenna array are Uniform planner array and you can do beamforming (move the beam) in vertical and horizontal direction (2D).
When N2 = 1 you have a uniform linear array and you can move the beam horizontally only (1D).
I think it is explained down in Sharetechnote page. I personally didn’t link those concepts together yet in my mind…
This post can help you understand this image in more detail, for the record this method is called grid of beams Precoding → Grid of Beams GoB precoding in 5G