Vector direction plot meaning

[CellRus]

Dear William,

I'm trying to look for some explanation on what exactly does a vector direction plot do but could not find it online so would love if you could explain it to me. 

I understand roughly that it plots the angle between vectors from -180 and 180. But what I'm unclear is what are the angles relative to? Are they relative to the average direction of the entire flow? Or are they relative to the surrounding local vectors?

Thank you very much

Hal