I was trying to join a friend's session on MW2 the other day and had this screen pop up. It clearly says at the top of this menu: "Modern Warfare 2 Membersh", in which we can confidently assume that the last word is "Memberships". If you'll notice at the bottom is an option to "Add Microsoft Points", clearly indicating that this was intended to be a menu for purchasing different levels of Membership, presumably memberships would have been offered in a variety of durations.