The full eligibility criteria for your study must be applied using the targeting attributes so that all participants who are sent to your study can complete it and receive the incentive.
It is very frustrating for participants to get unexpectedly kicked out of studies, and the pool has been designed to help you recruit only the specific sample that you require for your study. Any participants inappropriately screened out of a study should not have their submission rejected, and will require payment.
If you want to recruit a niche demographic that you can't obtain using the existing targeting attributes, you will have to make a custom initial screening study.
Here is an example of how to use a custom initial screening study:
You only want to recruit individuals whose favorite flavor of ice cream is chocolate
However, you could not simply publish your study, ask the question in your survey, and reject any participants who respond that their favorite flavor is vanilla or strawberry (or exit them from the survey)
As there is no existing targeting attribute related to participants' favorite ice cream flavors, you would instead have to run an initial screening study that contains the question: "What is your favorite flavor of ice cream? Vanilla, Chocolate, Strawberry...etc."
Once you've collected the data from this initial study, you will have a list of Prolific IDs that match your desired screening criteria (i.e. any participants whose response is "Chocolate") and can invite them to your main study using the targeting attribute: Advanced > Custom Allowlist.