Functional Abbreviation for Inst Expression in Turing's 1936 Paper
The first part of the disjunction is for stating that the next &quot;state/m-configuration&quot; of the machine is the one established in $Inst(q_i, S_j, S_k, L, q_l)$ (in case z takes as value the next cell of y', i.e., y), the second part of the disjunct is used to say that the part of the tape not scanned by the machine preserves its value.