For a while I thought "first class" was used to express a better service for a higher price. For example the first class seats in a train or a plane.
Naturally for a long time I came to believe the same meaning is carried over for mailing and postal services. However this seems to be misleading and doesn't appear to be the case.
Is there a general shared meaning agreed upon for the word "first class" or "first" when it comes to mail, maybe even a legal meaning, or is it some kind of non-standard word with no clear implications for which I need to carefully read the description on the postal service that uses such a term?