It is a cost-oriented theory which defines the region in which the optimal factor combination will lie.
Economic region of production consist of negatively sloped portion of all isoquants.
The ridge lines are the combination of points where marginal product (MPLK) of one of the factors is zero.
When production reaches a point where substitution between the factors becomes impossible (MPLK), the isoquant becomes positively sloping.
No rational entrepreneur will operate at a point outside the ridge lines (Region of Economic Nonsense).