That is to say, “what exactly is of interest and why” (Cronin, Ryan, & Coughlan, 2008, p. 38)
Keep in mind: Your research focus may evolve as you start looking for material.
Do a Literature Scan to assess the current state of the field.
Attend conferences to learn from others.
Brainstorm by yourself or with peers
Develop a Concept Map and let the ideas flow!
Part of the review process is deciding what studies fit your criteria and what do not. This process is made easier by developing a detailed appraisal process. The type of review you're writing will determine some aspects of your search criteria, but you need to think it through and be specific about your particular goals. And be transparent - even if you don't include a write-up of your criteria in your paper, you should still be able to explain why you chose what you chose should anyone ask.
Factors to consider:
But in deciding what to include/exclude, Heather Colquhoun (2016) cautions that "Limits must be consistent with the question asked." That is, don't have criteria that are so rigid that you can't actually answer your research question.
If your work has an external due date — such as for a thesis or dissertation, or journal or conference submission — you must take that into account when deciding your criteria. Some suggestions: