A firm uncertain whether a planned structure breaches a rule can describe it to sec staff and ask for relief. If staff agree, they issue a no-action letter saying they would not recommend enforcement on those exact facts. Whole market practices, including some ETF and securities lending mechanics, were built on such letters.
Two limits matter. The letter binds only the staff, not the Commission or a court, and it applies only to the facts described. Firms that stretch the facts lose the protection. Staff can and do withdraw letters when market practice drifts.
Reading old no-action letters is a good way to understand why a market structure looks strange: someone asked for permission decades ago and the answer shaped everything after.
Related: sec, sec-comment-letter, self-regulatory-organization