Many standard-form contracts already require the paying party to ring-fence retentions. For those ones, no further agreement is required other than that you will use the Scheme. Where there isn't any mention in your construction contract, we've built
Pre-Qualification Rule 3 so that once you have started to use us, both parties are required to continue to use us unless you mutually agree to stop. No further paperwork is required either way, however, because our Scheme Rules cover everything.