I guess it means wether your child can have 250, or 400 or 500mg of phenylalanine, no matter what the source of the protein is. As for the severity it reflects if she has mild, moderate or severe PKU. Some people have some enzymic activity, which even it is low, it allows them to eat some more protein than othes with the more severe form of the disease.
As for the test during pregnancy, sorry for my english but what I meant was that when you're pregnant they test the blood of the embryo and know wether it has PKU or not. They cannot do this if they don't know which mutation you're carrying. If you ask me, I wouldn't care to know it. But for some people could be important.