Alt is an abbreviation for "Alternate Account". Generally referring to a player that is very experienced who creates another PSN ID so they can play under an alternate account. The reasons for doing so are many, a few are
- their main account is high rank but they don't want to play in U.S. Expert servers (dislike Friendly Fire, missing Game Modes, no one else in them, etc), so they make an alt so they can play in Standard
- they want to enter Rookie servers so they can pound on less experienced players (some feel this is bad because it may discourage and drive new players away from Warhawk)
- they may want to enter Rookie servers so they can train a friend
- they may want a "disguise" so they can avoid someone, avoid being targeted by someone out to get them, or so they can target someone anonymously...etc.