On major departure days, certain regions choose to block sales to prevent too many travelers from traveling standing up or on the ground. A TER ticket does not guarantee having a seat, unlike a TGV inOui, Ouigo or Intercités ticket.
Regional trains are the only ones to still offer total flexibility of access, with the advantage that the price of tickets, not subject to yield management, remains the same regardless of or the moment you buy it. Germany and Austria seem to have found a compromise offering for a long time and paid reservations.