Before revision:  We account for this by constructing a model that assumes that each product requires a large number of non-tradable inputs, or capabilities, and that a country can only make the products for which it has all the requisite capabilities. Products differ in the number and specific nature of the capabilities they require, as countries differ in the number/nature of the capabilities they have.
After revision:  We account for this by constructing a model that assumes that each product requires a large number of non-tradable inputs, or capabilities, and that countries differ in the number/nature of the capabilities they have.