First class and Premium

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

Premium and first class are semantically related. in excellent topic. In some cases you can use "Premium" instead an adjective phrase "First class", when it comes to topics like better, superior, highest quality, most excellent.

Premium adjective - Commanding a large price.

First class and premium are semantically related. You can use "First class" instead an adjective "Premium".