The most direct understanding I have about it is this. It is all about suppression. Whatever drug gives you the most hormone suppression. Taking away the hormone that feeds the cancer. Unfortunately this usually means the most side effects. Here is a link to a study, at the bottom or the opening page, there is more reading. You can follow that as far down the rabbit trail as you have time for. Lol
Bottom line is, I believe your doctor just wants to make sure that whatever the number is, that it is going down.
Does that make sense?