Carat and First class

Carat noun - The unit of measurement for the proportion of gold in an alloy; 18-karat gold is 75% gold; 24-karat gold is pure gold.

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

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

Carat and first class are semantically related. Sometimes you can use "Carat" instead an adjective phrase "First class".