Why on the page 892 Shelah in his Proper & Improper forcing writes $N_n$ is a countable model of $ZFC^-$ (so $\in^{N_n}$ is $\in \restriction N_n$ but $N_n$ is not necessarily transitive)
I.e. what he means by $ZFC^-$ and how the non-transitivity of $N_n$ is used? And even, what he means by "transitive" ? Is it that $B$ is transitive iff $A \in B \rightarrow A \subseteq B$ ?