While in a raid or group will overwrite the "Target Nearest Friend" hotkey to target the lowest HP player in your raid (as a % of health).
If the lowest HP player has >90 Corruption or Martyrdom, will instead target second lowest HP player. Only if everyone else has full HP will it target the corrupted player.
If all players have max HP, will target a group/raid member with the tank role buff (i.e. in dungeon/raid instance groups) or if no character has the tank role, will target the player with the most HP in the raid (assuming that they are the tank).