First class and Uppermost

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

Uppermost and first class are semantically related. in best topic. In some cases you can use "Uppermost" instead an adjective phrase "First class", when it comes to topics like top, first-rate.

Uppermost adjective - Being at a point or level higher than all others.

First class and uppermost are semantically related. Sometimes you can use "First class" instead an adjective "Uppermost".