lowness vs lower rank

lowness and lower rank both are nouns.

Word	Noun	Adjective	Verb	Adverb
lowness	Yes	No	No	No
lower rank	Yes	No	No	No

As nouns, lower rank is a hyponym of lowness; that is, lower rank is a word with a more specific, narrower meaning than lowness:

lowness: a position of inferior status; low in station or rank or fortune or estimation
lower rank: the state of being inferior

Other hyponyms of lowness include inferiority, lower status, smallness.

lowness (noun)	lower rank (noun)
the quality of being low; lacking height	the state of being inferior
a low or small degree of any quality (amount or force or temperature etc.)
a feeling of low spirits
a position of inferior status; low in station or rank or fortune or estimation