I'm trying also to understand the mysteries of the hair engine.

I understand the "Hair Density" dial (as the name says) control the hair density that is measured in hairs per area unit. For example:

The "One sided square" measures 0.1 x 0.1 Poser Native Units (let me call it as pnu). That prop has only 1 poly.
Creating hair on this entire prop and setting the hair density to 10000 hairs/square pnu will generate:
0.1 x 0.1 x 10000 = 100 hairs.

The "Square Hi-Res" measures 0.7 x 0.7 pnu. That prop has many polys. Creating hair on this entire prop and setting the hair density to 10000 hairs/square pnu will generate:
0.7 x 0.7 x 10000 = 4900 hairs.

Any way, interested in any further explanations about this subject.

Thanks