3 or more Planes are said to form a family if either
The planes are collinear i.e. they pass through a common line of intersection
The planes are parallel to each other
Adding equation of 2 Planes always gives an equation of a Plane that belongs to the same family as of original 2 Planes.
Subtracting equation of 1 Plane from other also gives an equation of a Plane that belongs to the same family as of original 2 Planes. (except in case of 2 parallel Planes whose corresponding \(x\), \(y\) and \(z\) coefficients are same).