Man-to-man marking, or man marking, is a defensive strategy where defenders are assigned a specific opposition player to mark rather than covering an area of the pitch.
On Orders Screen (before or during the game), managers can select player(s) that main duty will be to man mark opponent player(s).
Such a player (man-marker) will be busy only on following the target-opponent. They will not do anything else - nor to run for the ball, nor to cross another players, nor to follow their position path, nothing except close following the target-opponent. They will run for the ball only if their target-opponent is running for the ball as well. And if their target-opponent is controlling the ball, they will attack him and try to stop him.
Man-markers will try to be as close as possible to their target-opponent. Better decisions-skill guarantees that man-marker will be closer to their target-opponent.
If their team is controlling the ball, man-markers will try to push their target-opponent behind, by organized offside until they reach the opponent half of the field. (This depends from their decisions skill).
These players will not be very useful for creating team attacks. Their main duty is destroying target-opponents moves, preventing their movement, taking the ball, passes and shots.
If their target-opponent is not on the field (or leaves the field), man-markers will follow their default behavior.
If their opponent is not in start-lineup, but enters later - man-markers will play normally until his entrance. After that they will catch and follow their target-opponent.
Man-markers will follow their opponents all over the field, doesn't matter on which position man-markers are, only matters where is the target-opponent. For example - if you man mark opponent wing, your player will follow that wing on both sides, a.k.a. if the opponent manager change the wing from right to left - the man-marker will follow him on the new position as well.