## 3 Background: The STP Model  
The Multi-Level Perspective (MLP) framework on sustainability transitions [37] is recognized as a main approach for the understanding of major system level shifts in social, economic and technological practices at a necessary scale to meet the present challenges of sustainability [38]. The MLP theory considers that transitions result from non-linear interactions at three levels: niche, regime and landscape [10; 37]. Niches are \"protected spaces,\" such as R&D laboratories, subsidized demonstration projects, or small market niches, where users have special demands and are willing to support emerging innovations that deviate away from existing regimes [10]. Regimes are formed by socio-cultural, user/market, science/technological practices and rules, referring to the \"meso\" context. Landscapes, on the other hand, refer to the external macro context, such as demographical trends, political ideologies, societal values and macro-economic patterns [10; 37]. Overall, regimes are embedded within landscapes and niches are embedded within regimes [37]. Transitions are characterized by the interaction between the three levels: niche-innovations build up internal momentum; whilst, changes at the landscape level create pressure on the regime; and, destabilization of the regime creates windows of opportunity for niche innovations [10]. This theory focuses on the development of radical innovations by producers and on the role of political governance, thus neglecting the potential of trajectories in consumption processes that are also of critical importance to orchestrate a system transition [38]. Despite this focus, the MLP associates the category of \"special users,\" described as early adopters that engage intensively in modifying and developing the innovation ready for market release, to processes of radical socio-technical innovation [38]. This is also supported by [PERSON] [39], who locates radical transformation of regimes in early niche markets, constituted by early adopters with different preferences than mainstream users, often willing to pay a higher price to particular benefits they gain from innovation. This type of approach is the core of GPP, SPP and PPI. [PERSON] and [PERSON] [15] demonstrated, by using an empirical case, the potential of public bodies to orchestrate system innovation by actively managing the required interdependencies between technologies, institutions and practices, thus establishing public procurement as an additional governance mechanism for the transformation of socio-technical systems. These studies demonstrate the relation between socio-technical transitions and consumption and specifically public procurement,providing ground to the development of a model that incorporates insights from GPP, SPP and PPI practices into the MLP framework. The STP Model, presented in more detail elsewhere [13] maps out the key factors that influence the transformation of socio-technical systems towards sustainability (Figure 1), integrating different approaches towards public procurement, i.e., GPP, SPP and PPI, building on [PERSON] multi-level perspective (MLP) [10; 37; 40]. The STP model is focused on public procurement organizations and addresses each of the three interdependent levels that are structuring the MLP framework: niche, regime and landscape. Each level is composed of one or more building blocks, each one having related key factors. In the MLP framework, the niche level provides the seeds for systemic changes, supporting the emergence of innovations [10; 37; 40]. Niche development can happen in special geographical locations or in specific domains, acting as a vehicle for learning, building new social networks (for instance producers, scientists, users, policy makers) and improving the innovation so that it gains momentum for diffusion to other niches or even replace dominant regime practice [41].