Of course, the original Rule 80, Rule 240 and the CAB do not exist any longer - but American Airlines still uses that terminology, as on the Reaccommodations - Rule 240/80 page. Reading the Conditions of Carriage and Reaccommodations pages can be most facilitative to your travel when irregular operations are encountered.

(Since this thread was not meant to be AA-specific, it has been relocated. But perhaps others would like to comment or post links to their predilect airlines' comparable rules?)