The adjectives are determiners that can be placed in two different positions within a sentence to modify or describe a person or a thing.
The possessive adjectives express the belonging of something to a person or an animal. We distinguish them by number and gender.
— “It is not her sandwich, it’s my sandwich.” = Her/my describe another element in the sentence (they are possessive adjectives).
♦ “It is not her sandwich, it’s mine.” = Mine replaces another element in the sentence (it is a possessive pronoun).
NOTE: We don’t use determiners before possessive adjectives, because they are already a kind of determiners.