What does "Heading Angle" mean? And what does it represents?

[asperatology]

According to the documentation, I don’t understand the remarks given:

Convert a direction vector into a ‘heading’ angle.

‘Heading’ angle between +/-PI. 0 is pointing down +X.

Can someone explain this to me? Is it representing the math circle rotation in which 0.0f is pointing down on a YZ plane? Or it’s basically FRotator.Pitch?

[asperatology]

Digging further, I am guessing that Heading Angle is literally the “heading” in “Heading, yaw, roll”?