First class and In a class all by itself

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

In a class all by itself and first class are semantically related. in first-rate topic. In some cases you can use "In a class all by itself" instead an adjective phrase "First class".

First class and in a class all by itself are semantically related. Sometimes you can replace phrase "First class" with "In a class all by itself", this phrases are similar.