First class and Upmost

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

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

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

First class and upmost are semantically related. Sometimes you can replace phrase "First class" with "Upmost", this terms are similar.