secondary vs second-string

secondary is a noun but second-string is not a noun.

Word	Noun	Adjective	Verb	Adverb
secondary	Yes	Yes	No	No
second-string	No	Yes	No	No

secondary (noun)	second-string (noun)
coil such that current is induced in it by passing a current through the primary coil
the defensive football players who line up behind the linemen

secondary (adjective)	second-string (adjective)
being of second rank or importance or value; not direct or immediate	being a replacement or substitute for a regular member of a team
depending on or incidental to what is original or primary
inferior in rank or status
not of major importance
belonging to a lower class or rank