Quote Originally Posted by LA_MERC_YellowDog View Post
Dog tag??
When you knife someone from behind, you take their dog tags. It keeps a list of all of the tags you've taken. There are about 100 total dog tags available for you to pick as the tags that you wear. Some you unlock by playing in the game and others are unlocked by promotions like these (or pre-ordering the game).

It's just another collectible in BF3.