sitter vs poser

sitter and poser both are nouns.

Word	Noun	Adjective	Verb	Adverb
sitter	Yes	No	No	No
poser	Yes	No	No	No

As nouns, poser is a hypernym of sitter; that is, poser is a word with a broader meaning than sitter:

Other hypernyms of sitter include model.

sitter (noun)	poser (noun)
a domestic hen ready to brood	a particularly difficult or baffling question or problem
a person engaged to care for children when the parents are not home	a person who poses for a photographer or painter or sculptor
an organism (person or animal) that sits	a person who habitually pretends to be something he is not
a person who poses for a painter or sculptor