On the sidelines in a NFL football game the ballboy wears a vest with the large letter 'K' on it. What does the 'K' stand for?

The 'k' stands for kicking, according to this article:

There are two sets of ball boys at NFL games. One ball boy that wears an X and one that wears a K. What is the difference?' Ryan replied, 'There are 36 game balls for each NFL game. 24 by X, that are broken in and watered down. Those balls are for normal plays. There are 12 balls for the ball boys that wear K. Those balls are brand new. When there is a kicking play, one of those balls goes in. I'm not a big fan of it.

ESPN, Wednesday 20-Nov-02

At the same time, representatives of the home and visiting teams can inspect the kicking balls (K-balls) that the officials received directly from Wilson Sporting Goods.

http://operations.nfl.com/the-game/gameday-behind-the-scenes/

The Ball

The home club shall have 36 balls for outdoor games and 24 for indoor games available for testing with a pressure gauge by the referee two hours prior to the starting time of the game to meet with League requirements. Twelve (12) new footballs, sealed in a special box and shipped by the manufacturer, will be opened in the officials' locker room two hours prior to the starting time of the game. These balls are to be specially marked with the letter 'k' and used exclusively for the kicking game.