Nonstandard satisfaction classes.
Fullness of M
A satisfaction class for a model M is e-full, if S decides all If for every \Sigma_e sentence (in the sense of M) \varphi with parameters either \varphi\in S or \lnot\varphi\in S.
Fullness of M, {\rm Full}(M)\} is the set of those e\in M M for which M has an e-full inductive partial satisfaction class.
It is easy to see that {\rm Full}(M) is a cut of M and, if {\rm Full}(M)>\omega then M is recursively saturated. Also if M is countable and \{\rm Full}(M) contains an element greater than all definable elements of M, then {\rm Full}(M)=M.\end{enumerate} \end{prop}
Kaufmann and Schmerl showed that there are completions T of PA, such that for every M\models T, {\rm Full}(M) contains no definable nonstandard elements.
Problem: Suppose {\rm Full}(M)=M, does M have a full inductive satisfaction class?
Reference: Kaufmann, Matt; Schmerl, James H. Remarks on weak notions of saturation in models of Peano arithmetic. J. Symbolic Logic 52 (1987), no. 1, 129–148.