tie-in vs fixing

tie-in and fixing both are nouns.

Word	Noun	Adjective	Verb	Adverb
tie-in	Yes	No	No	No
fixing	Yes	No	No	No

As nouns, fixing is a hypernym of tie-in; that is, fixing is a word with a broader meaning than tie-in:

Other hypernyms of tie-in include fastener, fastening, holdfast.

tie-in (noun)	fixing (noun)
a fastener that serves to join or connect	the act of putting something in working order again
	the sterilization of an animal
	(histology) the preservation and hardening of a tissue sample to retain as nearly as possible the same relations they had in the living body
	restraint that attaches to something or holds something in place