Requirement 2: explains the key similarities and differences among the reviewed 15 articles
This requirement means you need to identify the similarities and differences between the 15 articles themselves, not the similarities and differences between the 15 articles and your research topic. You can identify the similarities and differences based on factors that you believe are important in this research theme. For instance, for the theme of 6D BIM, the important factors could be year of publication, country of the first author, research topic (e.g. energy or waste or water or emission or ?), research methodology used by the paper (e.g. questionnaire or interview or computer simulation or realworld experiment or what?), used software (e.g. ArchiCAD or Revit or Rhino Grasshopper or Green Building Studio or DesignBuilder or what?), data collected by the paper, etc. As themes are different, you need to read the 15 articles and judge by yourself, what are important factors in this research theme. Then, you identify the similarities and differences based on these factors. For instance, regarding similarities, you may find something like 10 of the 15 articles all used Revit. Regarding differences, you may find something like the first authors of the 15 articles all come from different countries, suggesting the importance of this theme is globally recognised.