Second Crosshair in Cockpit mode

[Armchairpilot]

What's that second iron crosshair to the left of the normal gunsight in most airplanes? I mean in real life. It's never actually used for anything in the game.

Is it used as a backup if the main sight stopped working?

Also, while I'm on the topic, why are the gunsights in the russian planes so awful? The lines are so thick and their are way too many hash marks, it makes it difficult to see whatever you're shooting at because it's usually obstructed by the sight.