top off vs fill

top off is not a noun while fill is a noun.

Word	Noun	Adjective	Verb	Adverb
top off	No	No	Yes	No
fill	Yes	No	Yes	No

As verbs, fill is a hypernym of top off; that is, fill is a word with a broader meaning than top off:

Other hypernyms of top off include fill up, make full.

top off (noun)	fill (noun)
	a quantity sufficient to satisfy
	any material that fills a space or container

top off (verb)	fill (verb)
finish up or conclude	become full
fill to the point of almost overflowing	make full, also in a metaphorical sense
	plug with a substance
	fill to satisfaction
	eat until one is sated
	fill, satisfy or meet a want or need or condition or restriction
	assume, as of positions or roles
	appoint someone to (a position or a job)
	occupy the whole of