First class and Foremost

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

Foremost and first class are semantically related. in best topic. In some cases you can use "Foremost" instead an adjective phrase "First class", when it comes to topics like top, eminent, high-ranking, most excellent.

Foremost adjective - Coming before all others in importance.

First class and foremost are semantically related. Sometimes you can use "First class" instead an adjective "Foremost".