The vocabulary used to describe individual steps and subclassifications in PCET varies significantly in the literature. Below we will use the following. Proton transfer (PT) refers to the transfer of a proton in the sense of a pure Brønsted acid-base reaction. Electron transfer (ET) refers to the transfer of an electron in the sense of a redox reaction, which may be intramolecular and is commonly modeled using Marcus-Hush theory. Sequential PT-ET and ET-PT reactions require a (meta-)stable intermediate corresponding to either of the off-diagonal corners of the square scheme. If both ET and PT transfer proceed via a common transition state and hence a single kinetic step, this is labeled concerted protonelectron transfer. Notably, proton and electron can be transferred to or from different sites in a multi-site process. In contrast, the electron and proton traveling together from donor to acceptor is referred to as hydrogen atom transfer (HAT), where the electron is considered to reside in the H 1s orbital. We note that the terms 'coupling' and 'concertedness' do not have a unique definition in the litera-ture. They are currently used in the context of the energy-based square scheme, but a rigorous definition in terms of a potential energy surface (PES) is missing, even though it would be more intuitively accessible to chemists.