The designated hitter cannot be used for any other player but the pitcher (however, see below regarding different rules in amateur play). Use of the DH is optional. The manager must designate a DH prior to the start of the game; failure to do so forfeits the right to use the DH, and the pitcher must then take his turn at bat. The designated hitter may not play a field position and he may only be replaced by another player not currently in the lineup. However, the designated hitter may change positions to become a position player at any point during the game; if he does so, his team forfeits the role of the designated hitter and the pitcher, another player, or a pinch hitter must bat in the newly-opened spot in the batting order. The designated hitter could also become the pitcher, in which case the pitcher or a pinch hitter must hit when that spot in the batting order comes up again. Likewise, if a pinch-hitter bats for some other player (such as, hypothetically, the first baseman) and then remains in the game as the pitcher, the team would forfeit the use of the DH for the remainder of the game.

In addition, unlike other positions, the DH is "locked into" the batting order and no multiple substitution may be made to alter the batting rotation of the DH. In other words, a double switch involving the DH and a position player is not possible. For example, if the DH is batting fourth and the catcher is batting eighth, the manager cannot replace both players so as to have the new catcher bat fourth and the new DH bat eighth. Once a team loses its DH under any of the scenarios discussed in the previous paragraph, however, the double switch becomes fully available, and may well be used via necessity, should the former DH be replaced in the lineup.

If a pinch-hitter bats, or a pinch-runner runs, for the DH, he then becomes the DH.