The research agenda on health regionalization seeks to identify challenges that undermine its effective implementation in a solidary model. In this regard, researches have been prioritizing certain analytical units, such as regulatory marks, States, health regions and Municipalities that are a hub for medium and high complexity health services. In this context, this research sheds light on the peculiarities of a pool of actors that appears marginally in the investigations, but that plays an important role within regional policy dynamic: non-hub municipalities, which are not reference for health services within their region. Therefore, this research has two objectives: first, to structure a taxonomy on the challenges concerning the implementation of health regionalization; secondly, to apply it to non-hub Municipalities, in order to validate them as an analytical unit. The selected methodology to reach these objectives was a case study with non-hub Municipalities of two health regions of the state of Sao Paulo: Metropolitan Region of Campinas and Marilia Region The taxonomy proved to be effective in its use as an analytical research tool: (i) it guided the fieldwork, (ii) supported the comparison of perception among the interviewees and (iii) was central to the broader understanding of regionalization from macro-categories. On the second objective, it became clear that non-hub Municipalities face particular challenges. Thus, this pool of players can be validated as specific analytical unit to understand cooperative regionalization, and deserve special attention when formulating and implementing public policies. Furthermore, we reach to the conclusion that the current regionalization model presents considerable limitations that negatively affect non-hub municipalities.