First class and High-class

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

High-class and first class are semantically related. in better topic. In some cases you can use "High-class" instead an adjective phrase "First class".

High-class adjective - Of the very best kind.

First class and high-class are semantically related. Sometimes you can use "First class" instead an adjective phrase "High-class".