Mystic Barrier affects only what players and planeswalkers each player may attack with creatures they control. It doesn’t affect what players may be targeted by spells or abilities or other interactions.

Mystic Barrier doesn’t affect creatures that enter the battlefield attacking.

In some formats, you may need to disregard teammates sitting between you and the opponent seated nearest you in the chosen direction. In formats like Emperor, other rules may prohibit you from attacking that player.

If two Mystic Barriers are on the battlefield with two different chosen directions, you may not attack a player or planeswalker unless that player or the controller of that planeswalker is the opponent seated nearest you in both directions. (This happens most often in two-player games.)