Not too sure if there is an explaination in the bread bible about this or not, but I found the following on theartisan.net:
Q. What is semolina and how does it differ from durum flour?
A. Semolina is the coarsely ground endosperm of durum wheat. High in protein, it is used to make the highest quality pasta. It is also used to make couscous—a North African and Latin American dish which is quickly becoming a staple in North America. Durum flour is a by-product in the production of semolina and is used for American noodles, some pastas and some specialty breads.