We explore in this paper theoretical contributions that are related to Milgram and Kishino's Reality Virtuality Continuum by conducting a systematic literature review. From this study, we draw inspiration for our proposed mathematical formalization of combining multiple Reality-Virtuality Continua in a single, mixed reality experience. Also, we provide a definition for XR transition protocol. To complete our contribution, we discuss two potential examples that will exemplify our formalization and identify future work to be addressed.