Question

what is subjective function

Answer 1

In mathematics, a function f from a set X to a set Y is surjective, or a surjection, if for every element y in the codomain Y of f there is at least one element x in the domain X of f such that f(x) = y. It is not required that x be unique; the function f may map one or more elements of X to the same element of Y.

Answer 2

You have probably misread “surjective” which is a more technical term for “onto” (meaning that the range covers every element of the co-domain)