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

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

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?

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