A protocol outlines your objectives, planned methodology, and eligibility criteria. Developing and publishing a prospective protocol is considered a best practice in conducting a systematic review. Publishing an a prior protocol (in advance of the work) is a requirement for many publishers and a recommendation of others. Additionally, publishing a complete protocol provides the authors with a set of guidelines to follow while working on the initial review and facilitates reliable reproducibility post-publication.
Note: elements may vary by discipline
"The preparation of a protocol is an essential component of the systematic review process; it ensures that a systematic review is carefully planned and that what is planned is explicitly documented before the review starts, thus promoting consistent conduct by the review team, accountability, research integrity, and transparency of the eventual completed review." - Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement
Eligibility criteria, also known as inclusion/exclusion criteria should be established before beginning your review. Eligibility criteria may include the types of studies most relevant to your research question. For clinical fields, you may also want to define the population and include specifics related to the population data.
Have a clear, defensible reason for each criteria you include or exclude.
Librarian tip: Review existing systematic reviews and protocols on topics similar to yours to see what types of eligibility criteria other authors are using. This can guide you in developing your own.