What is Non-availability Approach?

Legal Definition
The non-availability approach (introduced by Irving B. Kravis, 1956) is a concept explaining international trade by the fact that each country imports the goods that are not available at home. This unavailability may be due to absolute unavailability (lack of natural resources), or to relative unavailability (goods cannot be produced domestically, or only at prohibitive costs). On the other hand, each country exports goods that are available at home.
-- Wikipedia