The index of an element in A that is supposed to be "close"
to the desired k-th order statistic. This information is used to
choose the pivot so that the chance to fall into the worst case
performance ($O(n^2)$) is minimized and the average case performance
is maximized.
Element in A that is larger than exactly k - 1 other elements of A.
References
Azzini, I., Perrotta, D. and Torti, F. (2023), A practically efficient
fixed-pivot selection algorithm and its extensible MATLAB suite,
"arXiv, stat.ME, eprint 2302.05705"