 Definition 3 (Set \(\mathcal{S}\) of admissible structures)  Let \(\mathcal{A}_o\) designate the intended interpretation of the observation language. Further, \(\MOD(A(\TC))\) designates the set of \(L(V_o,V_t)\) structures that satisfy the axioms \(A(\TC)\). \(\EXT(\mathcal{A}_o,V_t,D_t)\) is the set of \(L(V_o,V_t)\) structures that extend \(\mathcal{A}_o\) to interpret the theoretical terms, where the domain of interpretation may be extended by a domain \(D_t\) of theoretical entities.