First class and Stand-up

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Stand-up and first class are semantically related. In some cases you can use "Stand-up" instead an adjective phrase "First class".

Stand-up adjective - Following the accepted rules of moral conduct.

First class and stand-up are semantically related. Sometimes you can use "First class" instead an adjective phrase "Stand-up".