PRISMA definition: A systematic review protocol describes the rationale, hypothesis, and planned methods of the review. It should be prepared before a review is started and used as a guide to carry out the review.
This is especially important because a systematic review is a research method that can guide clinical decision-making. However, protocols can be useful for other review types as well.
Benefits to writing a protocol:
See the links below for examples of protocol templates.
Registries list reviews that are in progress. They are important because it lets others in the academic and research community know that a review on your topic is underway. It also provides a registry number for the review group when submitting the final paper for publication.
See the links below for examples of review registries.