What is the resolutions and rules for cross border ticket selling in airline industry?

Do not rely on IATA anywhere in the world to try to supply the answer to this question. They are the body responsible for the creation and policing of the rule, yet are unable to provide either a definition or an extract of the rule from any of their offices in Montreal (global HQ), Amsterdam (European HQ), Singapore (apac HQ) or Sydney. Simply hopeless.