To let members know your general criteria for approval, set your Approval Preferences on your profile. Many clients use this to let members know that they prefer requests from certain member types, are specifically looking for reviews from certain territories, or wish to explain more about what they are looking for from their reviewers.
If you wish to let members know about your criteria for approval concerning a specific title, the best way to do so is to add it to the notes section on the title details page. See an example of this kind of note below:
“Due to the overwhelming popularity of this title, we may not be able to approve all requests; it's really caught us by surprise! We are working through each individual request and considering each one on a case-by-case basis and we plan to approve more than we decline. Thank you so much for your amazing support and your well-thought out reviews to date, it really does mean a lot.”