First class and High-level

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

High-level and first class are semantically related. in eminent topic. In some cases you can use "High-level" instead an adjective phrase "First class", when it comes to topics like high-ranking.

High-level adjective - At an elevated level in rank or importance.

First class and high-level are semantically related. Sometimes you can use "First class" instead an adjective phrase "High-level".