Your wording is confusing . . .
Can i use the term "below the plane" for a point inside the plane,
i.e when Ax + By+ Cz + D < 0?
The book uses the term "inside the plane" and the web material which i have searched.
However still some people use the phrase "below the plane".
Is it same as "inside the plane"?
A plane is defined by an equation: .
Any point that satisfies the equation
. . is said to be on the plane.
With an inequality, like
. . we have all the points which are below the plane.
Your book should not use the term "inside".
There is a great difference between "on the circle"
. . and "inside the circle".
Besides, "inside" means "within its boundaries".
A plane has no boundaries ... it extends infinitely.