Rehmania, Making Herbal Medicines

Please, what is rehmania? Is it a herb or a root?
Also, what do they mean by 4 part, or 1 part of something? I am trying to make the fertility tonic which contains 4 part of rehmania, 1 part of astragalus root, 1 part of dong quai root, 2 parts false unicorn root, 3 part of wild yam root and 1 part vitex berries.
I've got everything except the one rehmania, could you tell me if I cook everything together and just drink it three times a day. I would very much appreciate it if you could shed some light onto what I am supposed to do.

Rehmania root (Rehmannia glutinosa) is a commonly used root in many Chinese formulas for its immune stimulating and tonic properties. In this formula, the parts refer to the amount of herb/root that is used and could mean 1 cup,1 half pound, 1 pound, 1 ton, 1 dump truck full, etc. By the looks of this formula, it is meant to be a decoction. So to make it, you can replace the word ?part? for ?cup? and as such the formula would read:
4 cups of rehmania root
1 cup of astragalus root
1 cup of dong quai root
2 cups false unicorn root
3 cups of wild yam root
1 cup vitex berries

Mix all the above dried herbs and place in a large jar. Then take 1 to 3 T of mixture and simmer for 20 mins in 2 cups of water. Strain and drink over course of day. Generally this is repeated daily for several weeks or more.