The Points (LP and RP), together with AX, form a defensive cup around the thrower. We refer to the Point who is marking the disc as the strong-side Point, and the other Point as the weak-side Point. In the diagram, the RP is marking the disc and so he is the strong-side Point at this time. If the disc moves to the other side of the field, the LP will probably mark the thrower and hence will become the strong-side Point.
So one Point (strong-side) will mark the thrower, and the other Point (weak-side) and AX will stand on the force side trying to prevent any throw forward on that side. In theory, this set up will prevent throws through the cup. MD marks the area just behind the cup. LW & RW mark the areas on either side of the field. DP marks anywhere behind that.
The job of Point requires a fair amount of fitness and good marking skills, as they are the only defensive players that should ever be marking the thrower. The cup tends to have to do quite a lot of running. The three players in the cup (LP, AX & RP) have to decide which way the cup is going to force the thrower. This will often be towards the middle of the field, although there are instances when always forcing to a particular direction (eg into a cross-wind), or forcing line is good.
AX has the task of coordinating the cup, defending against throws through the cup, and marking any receiver that runs into the cup (known as a popper). Coordination involves telling the points who will mark a particular thrower, how wide the cup should be spaced and which way to move. AX should also listen to calls from MD as to where poppers are. The AX can turn around a lot of the time and cover any man who is near the cup. MD should usually alert AX to warn them when a man is near the cup by saying "coming in".
MD covers the area directly behind the cup. This usually involves marking man-to-man on anyone that comes into this area, until they leave it or they run through the cup. Once a receiver enters the cup, they are the job of the Points or AX, and should be ignored. The other job of MD is to talk to the cup. If a receiver is coming into the cup, AX needs to be told to close the cup up on that side. The cup may also need to be told if it is too open or too close together.
Conceptually, the Wings have the simplest job on the field. They each have one side of the field to mark, and the aim is to stop passes up either of the sidelines. Usually, if a throw gets around the edge of the cup, the thrower will be looking upfield for the next pass. The Wing's job is to find and stop any cutters to that thrower. This may involve going man-to-man for a short while. If the player gets the disc, the wing should drop back (contain!) and let the points mark the thrower. It is not the Wing's job to mark the player with the disc.
DP has the task of intercepting any long throws put up by the offence. The job is usually not hard since if there is any wind it is difficult to huck accurately, and thus it is not often that the Deep comes into play. As a result, the main job of the Deep is to talk to the other players, primarily the Middle and Wings, and tell them where to go and who to cover.