Flamming...
you need a plane that passes through the point P(1,-2,3) and it is parallel to the plane XY.
to define a plane you need 2 things...
1 one point ...you have it
2 one vector that is perpendicular to the plane..
in your case....the plane XY is parallel to your plane and the vector v(0i+0j+1k) is perpendicular to the plane XY
therefore to define your plane get the point and the vector v and you are ok....