first class

definition: by first class conveyance
example: with first class accommodations
speech part: adverb
synonyms:
definition: the most expensive accommodations on a ship or train or plane
example:
speech part: noun
synonyms:
definition: the highest rank in a classification
example:
speech part: noun
synonyms:
definition: mail that includes letters and postcards and packages sealed against inspection
example:
speech part: noun
synonyms: first-class mail

How first class is being used?