That‘s a device called a "Magpull" designed to allow the magazine to be easily removed from a pouch and or attached to webbing, carabiners, etc. They‘re not used by the CF, though I have seen some people adding loops of paracord to the bottom of a mag for the same purpose. That system isn‘t a bad idea except the loop should be rotated 90degrees to be more effective for stacking on a carabiner. Even so, it seems not really necessary, most people just through empty mags down their shirt on an attack and sort it out on consolidation.