RA 13- Portfolio Margining

Really confused on this topic (p. 330)- so it says there are two types of margin lending- rules based and risk based. It uses the United States as an example of a rules based… you can borrow up to 50% of the value of the position to buy additional stock, however it later states that portfolio margining is an example of risk based rules and again uses the United States as an example again… how can the US be used as an example for both? Also, I thought you could borrow 100% on your positions in the US as margin rules require initial margin to just be 50% (e.g. I purchase $10,000 of GE stock and borrow another $10,000 to have a total position worth $20,000 which your equity is 50% because you borrowed 100% of your original funds put forward), so why does the text say you can only borrow 50%?