The phrase "unwritten rule" refers to a guideline or expectation that is not formally stated or written down, but is generally understood and followed by a group of people. Full definition
Whether you focus on social media marketing or just promote the profile of your company or even look for the ways to outsource marketing through social media, being aware of unwritten rules of turning likes into clients is your potent weapon. (oddrepublic.com)
That flexibility is maintained through a set of unwritten rules about how people interact with one another. (entrepreneur.com)
Because that scenario means that a member (s) of the Cardinals organization violated the most important unwritten rule in the world. (sbnation.com)