A lot of multiple criteria decision-making methods require the use of weights or importance of coefficients. Usually authors of the methods do not mathematically define these coefficients. Therefore, their methods are only heuristic. In order to successfully elicit and apply the relative importance of criteria, it is necessary to have a rigorous definition for the coefficients. In this paper a mathematical definition of the assertion 'a group of criteria A is more important than a group of criteria B with two sets of positive parameters' is given. Based on the definition the numerical relative importance coefficients are mathematically defined. The main objective of the paper is to demonstrate how to apply those notions in decision making in order to restrict the well-known Pareto set.