Tactically, players make decisions before they execute an action (e.g., a first five-eighth decides to kick, or decides to pass, then does it!).  A referee, having observed an action by a player, then decides whether it is right or wrong!
 
The referee uses his tactical awareness to determine whether an action is okay or not.  For example, when a player leaves his feet at the tackle, the referee 'decides' whether it had any impact on the play, before blowing the whistle.  The referee may decide to play on, play advantage, or penalise immediately.  These are all 'tactical decisions', and a referee is required over time to build his 'game knowledge', so that he or she can make relevant decisions to the game.