This clever idea of Bob Bernecky's  provides a performance improvement for certain expressions by marking certain arrays with special properties. For example, the property of being a Permutation Vector  is invariant (is still a Permutation Vector, albeit a different one) under various APL primitives such as rotate/reversal (L⌽PV and ⌽PV) and grade up/down (⍋PV and ⍒PV).
Bernecky has defined several array predicate properties, one of which has been implemented in NARS so far.
In this case, index generator (⍳R) produces a Permutation Vector, as does deal (L?R) when the left and right arguments are the same — the results of these primitives are marked internally as Permutation Vectors. Subsequent use of such arrays maintains that property when operated on by rotate/reversal and grade up/down. Moreover, the two grade (⍋PV and ⍒PV) and the index of (PV⍳R) primitives use a much faster (linear) algorithm than they would normally, given that the appropriate argument is a Permutation Vector.