## 3 Background: The STP Model
In Strategic Niche Management (SNM) theory, success or failure of a niche can be explained by analyzing the interactions between three main niche processes: (i) shaping of expectations--articulating expectations and visions in order to attract resources and new actors and provide direction to the process; (ii) building social networks--new combination of actors, in order to promote the emergence of new social networks; and, (iii) learning processes--social embedding to increase chances on successful diffusion [41; 42]. In the STP model, the niche level is characterized by the organization's internal practices (organizational factors), as well as by external practices concerning its relationships with others, which are required for spreading innovation (inter-organizational factors). The organizational factors block is divided into three categories: (1) individual factors, accounting for the role of the individuals within the organization and encompassing two key factors (change agents and knowledge); (2) management factors, referring to the organization's culture, strategies, structures and practices required for supporting GPP, SPP and PPI and encompassing five key factors (organizational culture, strategy, internal change, internal collaboration and responsibilities);  
Figure 1: The Sustainability Transition Procurement Model (STP Model). and (3) procurement factors, which focus on practices that support GPP, SPP and PPI, encompassing the following key factors: aggregate demand, market research, monitoring, SME (small and medium size companies) participation, LCC (Life cycle costs), risk management, functional criteria/variants and IPR (intellectual property rights) management. The inter-organization factors block relates to implicit knowledge located within collective interactions, which can be oriented towards suppliers, similar organizations and/or other external stakeholders. Accordingly, this block encompasses four key factors: suppliers' involvement, networking, external stakeholders and vision. Moving from GPP to SPP to PPI, the organization increasingly needs to: develop earlier and longer-lasting collaborative activities with suppliers [43]; work in networks with similar organizations that use each other as reference points, sharing best practices, promoting interactive learning and spreading knowledge [44; 45]; and, engage with other external stakeholders, such as managers in public agencies, policy-makers, public procurers, potential users, suppliers, researchers and non-governmental organizations [3; 15; 43]. These relationships are addressed in the key factors suppliers' involvement, networking and external stakeholders. Furthermore, inter-organizational interactions spread knowledge and best practices to other actors, creating a common vision [15; 46], hence contributing to change the existing regime--this referring to the STP model' vision key factor. Thus, the three main niche processes described above (shaping of expectations, building social networks and learning processes) correspond to the inter-organizational building block. The multi-faceted relationships and on-going iterative engagements can shape local and national strategies and align policies at national and local levels, thus influencing the regime level, as demonstrated by [PERSON] and [PERSON] [15]. In the STP Model, as in the MLP perspective [37], the regime level consists of the routine-based behavior of organizations and of other actors involved, encompassing two key factors: policies/regulations and policy guidance. Thus, the regime level includes short to medium term policies and regulations which favor the introduction of environmental, social and innovation considerations in public procurement. The European Directives on public procurement [47] constitute an example, allowing for environmental and social criteria and defining new procurement procedures such as the \"innovation partnerships,\" which acknowledge a strong link between procurement and innovation [48].