reusable routine vs subprogram

reusable routine and subprogram both are nouns.

Word	Noun	Adjective	Verb	Adverb
reusable routine	Yes	No	No	No
subprogram	Yes	No	No	No

As nouns, subprogram is a hypernym of reusable routine; that is, subprogram is a word with a broader meaning than reusable routine:

Other hypernyms of reusable routine include function, procedure, routine, subroutine.

reusable routine (noun)	subprogram (noun)
a routine that can be loaded once and executed repeatedly	a set sequence of steps, part of larger computer program