fill vs rack up

fill is a noun but rack up is not a noun.

Word	Noun	Adjective	Verb	Adverb
fill	Yes	No	Yes	No
rack up	No	No	Yes	No

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

Other hyponyms of fill include water, flood.

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

fill (verb)	rack up (verb)
become full	supply a rack with feed for (horses or other animals)
make full, also in a metaphorical sense	defeat thoroughly
plug with a substance	gain points in a game
fill to satisfaction	place in a rack
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