First-rate and First class

First-rate adjective - Of the very best kind.

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

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

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