“…It is not possible to join only one or the other pillar of the BU, only the BU as a whole. This means that BU represents both the multi‐speed and variable geometry differentiation, but not à la carte differentiation (Ferran, 2017; Zeltin and Rangoni, 2022). The motivation for opt‐in and opt‐out, that is, the reasons for horizontal differentiation, is a well‐researched topic (e.g., Darvas and Wolff, 2013; Howarth and Quaglia, 2016; Mérő and Piroska, 2016; Montanaro, 2016; Spendzharova, 2014).…”