First class and Highest

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

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

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

First class and highest are semantically related. Sometimes you can use "First class" instead an adjective "Highest".