It’s mostly a CAD thing.
For instance you are working on a complex space ship and want to add a cargo door to a rounded hull.
You would make a working plane that fixes on the hull so that any modifications would be perpindicular to the hull even though the hull is round in shape.
Think of it as a magnetic “Rubic’s Cube” that allows one to keep things perpindicular to the object they are working on in object mode only.
(X,Y) (X,Z) (Z,Y) It will allow one to zero out one of the X,Y,Z axis so that the plane can be used to precisely align you work in two axis only.
Symmetry works this way - sort of.
It’s also why robotic controlled arms can work at weird angles on parts. They use custom working plane geometry to keep two axis control so that the object is always aligned in object space rather than world space.
I’ve never used this in Hexagon, don’t know where it located or how it is used. heh
Here’s an autocad tutorial that demonstrates one use.