Offer clear definitions on rules for rejection rates on Prime Panels. Currently the site (and communications with support) say "Please note the maximum rejection rate is 30 %." What I learn when using prime panels in practice is that this means 30% of the size of the sample you requested, not a rate of people taking the survey that you reject, i.e., denominator for calculating the rate is x instead of (x + # rejected)

Since this affects strictness of the screens you can use, it's important to make it clear up front so users can specify a rule they will be able to apply throughout.