Ahh :razz: I know a better definition for a plane:

(x,y,z),(nx,ny,nz)

It's a point in space + a plane normal.
My first definition was not completely waterproof because it couldn't contain a plain which runs parallel to the Z-axis. My new definition can.
It's also i bit more compact than yours.

I think.. to transform a plane using this definition, you must multiply the matrix with the (x,y,z) vector and divide the normal vector with the matrix and normalize it afterwards.

Is this a correct way?? can someone confirm this.