Tag skill

Tag skills are the skill specializations of a character. At the start of every core Fallout game prior to Fallout 4, the player can choose three Tag skills to be the character's specialties throughout the game. The Tag! perk allows one to pick an additional, fourth tag skill later in the game.

In Fallout, Fallout 2 and Fallout Tactics, tagged skills increase at double the rate of an untagged skill (i.e. One skill point translates to a 2% increase in the skill). In Fallout 3 and Fallout: New Vegas however, a tagged skill only gives a 15 point boost.

Tag skills are a vital part of the Fallout universe because they essentially dictate what type of character one will play. A fighter will tag combat skills whereas a diplomat may choose speech, barter and science. A well-rounded character usually includes at least one diplomatic and one combat skill.

Bonus equipment
Tagging skills also provides an immediate benefit to one's starting equipment in Fallout, Fallout Tactics and Fallout: New Vegas.

Fallout

 * Tagging both Lockpick and Steal results in only one bonus set of lock picks.

Tagging Big Guns, Energy Weapons, Melee Weapons, Sneak, Traps, Speech, Barter, or Gambling skills award no additional bonus equipment.

Fallout Tactics starting equipment

 * 1x MP5 H&K
 * 48x 9mm ball
 * 1x Happy Pie
 * 1x Classic Nuka-Cola
 * 1x stimpaks
 * 1x Midwest Brotherhood leather armor

Fallout: New Vegas starting equipment
After completing the tutorial, every character gets:
 * 4 stimpaks
 * 18 caps
 * 6 bobby pins
 * possibly one ranged weapon (9mm pistol is default if no weapon skills are selected) or one hand-to-hand weapon.

Behind the scenes
At one point, it was planned for Fallout 3 ' s tag skills to give double the points, the same as the previous games.