The A, B, C and D are for the plane equation: Ax+By+Cz=D. This defines a plane completely, and takes up only 4 floating point numbers.

A plane can also be defined with an origin (Point3d) and a normal (Vector3d), or by choosing one of the pre-defined planes: WorldXY, WorldXZ or WorldYZ.

@dale Sorry for the bad explanation but I was searching for the answer Menno gave @menno Oh the point3d and normal vector3d was the second thing I was searching for xD
I know how to work with vector and point3d origin, but before i just played around with abcd and filled in some numbers till I got the result I wanted xD