First class and Unbroken

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

Unbroken and first class are semantically related. in good topic. In some cases you can use "Unbroken" instead an adjective phrase "First class".

Unbroken adjective - Going on and on without any interruptions.

First class and unbroken are semantically related. Sometimes you can replace phrase "First class" with "Unbroken", this terms are similar.