"it is really quite simple. For everything other than Main Panel A (medicine and related), pedagogic research can be submitted to any subject panel, which will then take advice from the Education panel".
The REF2021 Draft Guidance for Panel Criteria and Working Methods covers this: search within the PDF for "pedagogic" to see how the different panels address it: https://www.ref.ac.uk/publications/consultation-on-the-panel-criteria-and-working-methods-201802/