In the rest of the world a Clip is called a Magazine Charger, or Charger for short. That's what it does, charges (fills) the magazine.
A clip for the M16/AR15 is used to -- you guessed it [;)] -- charge the removeable magazine. On this rifle an adapter is fitted to the magazine, the clip inserted, the bullets pressed out of the clip into the magazine, the clip is removed and discarded. The adapter is removed and the magazine is then inserted in the rifle.
On other rifles, the SKS, M1903, Mauser, etc are a good examples, the magazine is a permanent fixture in the rifle and the clip (charger) is used to -- again [;)] -- charge the magazine while it's still in the rifle. Clip is discarded after charging the magazine. Rifles like the M14 can have their magazines charged in or out of the rifle.
On still other rifles, the M1 Rifle (Garand) being a good example, the clip is used to charge the permanent magazine in the rifle. No surprise here. In this case, however, the clip remains in the rifle until it's ejected with the last fired cartridge.
In all cases the clip is used to charge the magazine.
-- Chuck