## 2 Theoretical Framework
### Voluntary Standards in Green Public Procurement  
The use of voluntary standards and eco-labels to support the procurement of sustainable solutions in GPP are means by which sustainability missions can be incorporated in practice. In addition, a demand-side innovation mechanism, the process of standardization establishes criteria beyond that which is required by law (if any), where the particular process contributes to the resulting standards' legitimacy and credibility. Similar to missions and GPP, it engages multiple stakeholders, including firms and technical experts, toward increased levels of performance (more relevant to when the state of the art for a particular area of focus is at late-stage R&D, or once the product or service has been commercialized onto the market). The voluntary standards resulting from standardization can be of many different types, often with a direct connection to the degree of technological and market maturity of a product or service being standardized (or the R&D process leading up to it). For example, standards for measurement, testing, and architecture can help guide the R&D process such as in a pre-commercial procurement (PCP) for more radical innovation [10]. For more incremental innovation such as through the public procurement of innovative solutions (PPI), variety-reducing standards can be used to limit horizontal product differentiation [32], whereas quality standards assist in requesting a minimum performance or desired functionality [10]. In comparison, and most relevant to off-the-shelf products or services and their diffusion effects on innovation, eco-labels are based upon information standards or product description standards [33] and convey information about a product or service to the consumer regarding environmental attributes [10]. Eco-labels can also be requested by procurers for suppliers to communicate quality attributes. Not only the type of voluntary standard, but also how it is used, will influence the innovation and sustainability impacts of GPP. In tenders, including those for SUD projects, voluntary standards can be used in a number of locations and manners [34; 35]. Methods for designing reward systems within the tender itself to facilitate sustainability performance in accordance to the voluntary standards include (1) requiring a technical standard within technical specifications that mandates adherence to the standard for the proposed solutions to be eligible, such as those specified in a municipal sewer plan, and (2) awarding points within the award criteria based on performance of the proposed solution to a particular standard, which could be made to be either mandatory or voluntary and the points system adjusted accordingly, such as efficiency specifications of the Nearly Zero-Emission Building (NZEB) standard. Additionally, incorporating voluntary standards into (3) contract clauses, such as sustainability performance standards for increasing emissions efficiency, is also becoming a more popular choice to share risks and benefits between procurers and suppliers over the course of what is usually a multi-year contract. Depending on the public procurement procedure used (open, closed, competitive procedure with/without negotiation, etc. ), standards can also be used to define (4) increasingly strict selection criteria for groups of candidates between phases, such as pre-selection being on the basis of 35% \"sustainability\", and the final selecting on the basis of 45%, with sustainability being clearly defined by specifications. Taken together, the set of voluntary standards used should orient the tender toward achieving the sustainability missions relevant to the contracting authority. Management literature on the topic, focusing instead on influence of and on the supply-side with public procurement, has investigated the uptake of voluntary standards in connection with public tenders. Such investigation can be used to study both direct and \"ripple\" effects of GPP and innovation procurement, including wider market impacts and changes.