Offside is a law in association football which states that if a player is in an offside position when the ball is touched or played by a teammate, he may not become actively involved in the play. A player is in an offside position if he is closer to the opponent's goal line than both the ball and the second-to-last defender (which is usually the last outfield player), but only if the player is on his opponent's half of thepitch. "Offside position" is a matter of fact, whereas committing an "offside offence" occurs when the a player is "actively involved" and is subject to the interpretation of the referee. Goals scored after committing an offside offence are nullified if caught by the referee
Agreed; it's up to the referee to interpret whether the player is actively involved or not. The game simulates real life in sometimes allowing and sometimes not.