What is the significance of N1 and N2 in codebook?

Hi Experts,
What is the significance of N1 and N2 in codebook?

It gives the number of beams in horizontal and vertical directions.
N1xO1 is for horizontal and N2xO2 is on vertical direction.
For grid of beams.

Please read this short article: Types of beamforming in 5G: