People are becoming increasingly aware of the importance of sustainability. Indeed, sustainability has become more and more important to businesses, governments, charitable foundations and, also, for the software industry. As a result, the areas of convergence between software and sustainability have gained in prominence and a focus on two different perspectives has been proposed: software sustainability (SOS) and software as part of sustainability (SAPOS). In this article, both perspectives are explained and differentiated, and the dimensions of each one are presented. Considering the global importance of sustainability, the UN has proposed 17 Sustainable Development Goals to help protect the planet. We analyze the interactions between these goals and the dimensions of SOS and SAPOS, and find that the latter, being a more general concept, is related to more of these UN goals. Nevertheless, software plays a fundamental role on both perspectives that, albeit different, are complementary and equally necessary.