Reference Markers are a new type of waypoint indicators (introduced in V3.4) that can exist parallel to regular waypoints, and can be assigned to either a specific unit or a vehicle.
They appear as a cross (or any custom designed icon), and stay at the same size, no matter the distance to the player. Underneath the icon the current distance is shown. If the mark's position is out of the view of the player, then an arrow at the edge of the screen indicates the relative direction.
Unlike waypoints, reference marks are purely visual entities, and have no automatic functionality attached to them (i.e. no actions can be assigned to them, and they don't disappear when the player is within a certain range).
Their format is similar to that of waypoints, in that it exists of a 2-element array, containing [side,index]. The same mark can be assigned to different entities, but each entity can only have one mark (any subsequent assignments will overwrite the previous one).