What is a safety in football, i.e., 2 point score?

For a safety it depends where their knee or elbow touches the ground. It doesn't matter where the ball is,all that matters is where they touched down first.It's like a tackle on the field the ball is spotted where the player's knee or elbow touches down. Only when scoring a touchdown and getting the first down line is where the ball matters to see if crosses a plane. On a safety the ball rule is not added. So the ball can be outside the plane but if the player's knee is in the endzone it's a safety.