fill vs top off

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

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

As verbs, top off is a hyponym of fill; that is, top off is a word with a more specific, narrower meaning than fill:

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

fill (verb)	top off (verb)
become full	finish up or conclude
make full, also in a metaphorical sense	fill to the point of almost overflowing
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