## 4 Discussion
Further research was also proposed to assess the level of sustainability contribution, the competences covered and developed, and the pedagogical approaches used in one or more HEIs at all levels of education [137; 123]. Furthermore, our review revealed a lack of indicators to assess and measure the integration of sustainability and SD in education and research [80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 164; 165; 166; 167], as well as in SD through on-campus experiences [158; 159]. The category \"outreach and collaboration\" included studies that examined sustainability-oriented HEI networks as well as networks between HEIs and communities or local or regional institutions. Sustainability-oriented HEI networks play an important role in the sustainability transition [8; 145; 146; 147; 148; 149], yet fewer articles were assigned to this category. Additionally, no comparative assessment of different HEI networks has been conducted [8; 145; 146; 147; 148; 149]. Our review of the articles from the category \"assessment and reporting\" implied that SATs may not fit the local needs, and developing a tailored model can be more advantageous for a country or HEI [168; 169]. These implications are aligned with the implications from the category \"institutional framework,\" namely, organizational management tools should be adapted to HEIs' unique mission and impacts [35; 111]. The analysis of the articles from the category \"assessment and reporting\" led to similar implications as stated before: HEIs' sustainability impacts are not fully recognized, and accordingly, the sustainability progress of HEIs is not clearly measurable [6; 164; 165; 166; 167]. SATs neglect the impacts that HEIs have outside their organizations, mostly focusing on operations [6; 164; 165; 166; 167]. A similar gap as before was also identified here, namely, one of the most urgent research topics proposed was the selection and development of indicators to measure the sustainability performance of HEIs, especially regarding research and teaching activities [191]. Researchers need more information about how to properly assess HEIs' contribution to sustainability; this lack of knowledge represents a clear barrier to SR [190]. This research has several limitations. The literature search was conducted in March and April 2020; therefore, research conducted after this period was not considered in this literature review. Several exclusion criteria were defined prior to conducting this literature review, and using another set of exclusion criteria could lead to slightly different results in the final sample. Previous research [3; 5] indicates that the HEI system consists of seven elements, which were used for the categorization of the final literature sample. In some cases, making a clear distinction between categories was difficult for several reasons, e.g., the categories were interrelated and the analytical results in one category were used or implemented in another one. For example, some articles were on the HEI network for SD in education. The category could be \"outreach and collaboration\" in the sense of the HEI network, or \"education\" in the sense of SD in education. Similar examples include articles on the effects of online learning on campus operations (\"education\" and \"campus operations\" categories) or articles on research on SD education (\"research\" and \"education\" categories). When the categorization was unclear, the article was assigned to the category according to its prevailing elements; however, this could have resulted in a certain author bias.