Does anyone know the units for the number associated with the Hair Density dial in the Hair Room? The space for the total number of hairs is too small to display the number and I can't seem to give it more space. The Hair Density would help if I knew it was per square what. Poser Pro 2010 if that makes any difference. Thanks.

Whichway

I am 90% sure that all dynamic weights are per vertice

 It depends on the mesh they're grown on. A very dense /high res mesh will give you more hairs than a low rez mesh. So if you want more hairs, subdivide your mesh :)

The number of hairs does not depend on the mesh density. For a simple proof load up the low-res and high-res ball primitives and grow hair on them.  With the same density settings they grow the same number of hairs.
As far as I can judge, the density figure appears to be in hairs per square inch

Aha. I was mislead by the number of guide hairs, those are a LOT more sparce on a low rez object.

Hm then I've got a problem. I am working on a sort f skull cap and in one area the hair is too dense. It coincided with a place on the model that has very high polygon density and I thought that was the reason. But now it seems like it can't be.

pulls out hair, both real and virtual

You could select a few individual guide hairs in the area and dial the length down to very short.

 I've actually made sure not to select any of the dense polygons in the hair group at all.. and still there's a dark, hairy spot L 

But thanks for the idea, I might try to manually style the hair in that spot and see if it helps.. it can only get better L

Turns out with some experiments and a look into the geometry files, the density is in grown hairs per square display unit, independent of the polygon density. So, if your Preferences set the display units to feet, it's hairs per square foot; centimeters, then hairs per square centimeter. What is actually stored in the file is hairs per square Poser Unit, i.e., per (8.6 feet)**2. Rather a lot, usually. [17,000,000 for average human hair.] The total number of hairs value is correctly given and is the density multiplied by the area of the hair group, which, unfortunately, is not shown in the UI, but is contained in the file in square Poser Units, however, it does not include guide hairs, only grown hairs, so the actual number of hairs is larger. For a fine mesh and low hair density, the guide hairs can actually overwhelm the grown hairs. [Been there, done that.] At least that part of the Hair Room now makes sense to me.

Whichway

P.S. - If you change the Preferences display units on the fly, you probably have to at least close the document and reopen it to get the density calculation updated.