MED is short for "Medium" and refers to the length of the rifle action this magazine is compatible with. The majority of actions are either short or long action but there has been a growing desire for something in between. These magazines work with the Medium, XM and XM Plus chassis and bottom metals available. They are great for people who want to run either a really long short action caliber that doesn't fit in a short action mag, or a short long action cartridge that may have trouble feeding in a standard long action magazine.