Ace and First class

Ace adjective - Having or showing exceptional knowledge, experience, or skill in a field of endeavor.

First class and ace are semantically related In some cases you can use "First class" instead an adjective "Ace".

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

Ace and first class are semantically related in excellent topic. Sometimes you can use "Ace" instead an adjective phrase "First class", if it concerns topics such as good, pleasant, first-rate, super.