One of the features that distinguishes a systematic review from a narrative review is the pre-specification of criteria for including and excluding studies in the review (eligibility criteria). Explicit criteria, based on the review’s scope and question(s), are used to include and exclude studies.
A large number of references (study titles and abstracts) will have been found at the searching stage of the review. A proportion of these will look as though they are relevant to the review's research questions. So, having explicit criteria against which to assess studies makes the process more efficient in terms of time.
More importantly, it also helps to avoid hidden bias by having clear consistent rules about which studies are being used to answer the review's specific research questions.
Each study needs to be compared against same criteria. To be included in the review, a study needs to meet all inclusion criteria and not meet any exclusion criteria. Inclusion/eligibility criteria iclude participants, interventions and comparisons and often study design. Outcomes are usually not part of the criteria, though some reviews do legitimately restrict eligibility to specific outcomes.