Question about Hoya Pol. Circ. Filter Pro 1 Digital

Found a good explanation on this page:
http://www.spotcatch.net/foto-blog...

"....
The Hoya HD costs about 20% more than the Hoya Pro1, the reasons for this are that the Hoya HD has the following features compared to its cheaper counterpart:

Nano-coating >(easier cleaning)
Tempered glass >(more scratch resistant)
Higher transparency >(more translucent)
..."

I don't know the difference either. You would have to have both filters or be able to test them. I assume HD means high density and should eliminate reflections even more. Personally, the "normal" polarising filter is sufficient for me and I think it has a good quality. For photos, e.g. through an aeroplane window or a shop window, it is definitely sufficient.

I have the "Pro 1 Digital" filter and unfortunately I don't know what exactly the difference is. I would also be interested.