Scientific studies give importance to the stakeholders' engagement. It is very significant issue on sustainability in the enterprises. From the Fuzzy Logic and Stakeholder Theory, it aims to explore the sustainability and to incorporate the Global Compact principles into strategies for sustainable purchasing. It shows a conceptual study that attempts to know the degree of contentment of the code of conduct on the part of the providers and the usefulness of an instrument that helps the firm's decision-making process on the topic. We will proceed through a model that allows certain levels to the homogeneous group of variables. The basic components of decision theory are used, particularly the theories of relation, such as the affinities in the families of Moore and its illustration through the Galois lattices. As results we will provide contributions to the Affinities Theory so as to get to the approaches for sustainable purchasing.