Polo Rules

Assuming you were querying the 'man marking not screening thing':

Yes.

When man marking you track players rather than screen them. If you stop the non-ball-carrying offensive player's movement then you will be penalised for obstruction or interference.

Man marking enables defenders to stay in a good position to intercept or challenge when their man is passed to or receives the ball. They are not looking to obstruct as it illegal.

Red and Blue both playing 2-out.

Red has possession, on the edge of Blue's D.

Red 1 is facing Blue goal, Red 2, Blue 1 and Blue 2 facing Red goal.

Red 1 attempts a pass to Red 2. Blue 1 intercepts.

If Blue 2 rides parallel to the fence such that Red 2 is restricted to a 1m channel along the side fence, is that marking or screening and if it's screening, how do you turn it into marking?

This feels like a common blocking move that means Blue 1 is one-on-one with goalie Red 3, whilst Red 1 is chasing, and will probably hope to cleverly hook if Blue 1 goes for a big wind-up.