Sorting is when the constituents, align with the party around a given issue. If Democrats are all yes, Republicans are all no, and undecideds are split evenly, that is a perfectly sorted. Sorting, crucially does not necessitate a change in opinion amongst the constituency. Polarization occurs when their is sorting and the undecideds begin to drift towards either pole/party1.
1. Klein, E. Why Weâre Polarized. (Avid Reader Press / Simon & Schuster, New York, 2020).