Can you define an endpoint of a plane

Yes. The endpoint of a plane can be defined.

Consider the example of a wall in the room. There are 3 planes. The x plane i.e. horizontal plane. The y plane i.e. vertical plane. The z plane which is the plane directed outwards from the wall.

These 3 plane meets at the start/end of a wall i.e at a corner. The corners of the room are the starting and the ending points of planes in this case.

Therefore, in this case we can define the corner as the endpoint of a plane. The coordinates of the corner or endpoint of plane can be calculated by measuring the dimensions of the room.

Such planes have finite spread. There are also planes which do not have any end and they spread indefinitely in all direction. The endpoints can only be defined for planes with finite spread not for the infinite planes.