expansion vs subjoining

expansion and subjoining both are nouns.

Word	Noun	Adjective	Verb	Adverb
expansion	Yes	No	No	No
subjoining	Yes	No	No	No

As nouns, subjoining is a hyponym of expansion; that is, subjoining is a word with a more specific, narrower meaning than expansion:

Other hyponyms of expansion include amplification, elaboration, annotating, annotation, subjunction, supplementation.

expansion (noun)	subjoining (noun)
the act of increasing (something) in size or volume or quantity or scope	the act of supplementing
adding information or detail
a discussion that provides additional information
a function expressed as a sum or product of terms