In the typical foundations of Zermelo–Fraenkel set theory, semisets are impossible due to the axiom schema of specification.
It is based on a modification of the von Neumann–Bernays–Gödel set theory; in standard NBG, the existence of semisets is precluded by the axiom of separation.
The concept of semisets opens the way for a formulation of an alternative set theory.
In particular, Vopěnka's Alternative Set Theory (1979) axiomatizes the concept of semiset, supplemented with several additional principles.
Novák (1984) studied approximation of semisets by fuzzy sets, which are often more suitable for practical applications of the modeling of imprecision.