The global environmental crises continue to get worse, fast approaching various irreversible thresholds. While a vast array of approaches to solving sustainability problems are found under the umbrella of Sustainable HCI, their contributions are sometimes hard to compare. In this essay, we describe a set of assumptions that influence what is considered meaningful and important areas of sustainability research, along four dimensions of sustainability: 1) the depth and nature of the sustainability challenges; 2) the role of technological innovation in sustainability; 3) what gets defined as "externalities"to a design or system; and 4) the time perspective used to consider sustainability. We argue that what one assumes within each of these dimensions directly influences what one means by the term "sustainability", which is then reflected in the questions that are asked, the methods chosen, the proposed solutions and the developed systems. By describing these assumptions and some of their commensurate actions, we offer a framework that may enable members of the SHCI community to reflect on and better position their own work and that of others in the field. Our intention is for the framework to lead to better transparency and more constructive conversations about where we might collectively direct our efforts moving forward.