Explain the rule of substitution in the game of hockey.

Rule of substitution in hockey: 1. A team may use up to 5 substitutes and make unlimited substitutions during the match. 2. A substitution can be made at any time, except during a penalty corner after the ball is in play and during a penalty stroke. 3. The substitute enters the field only after the player being replaced has left it, near the centre line, and with the knowledge of the umpire. 4. A goalkeeper can also be substituted.