It does mean that they have stopped selling Y and PE so they are full.
Upper could also be full but they are happy to sell tickets at a price as they can probably make more money by selling the ticket and then offering people compensation to downgrade or move to another flight.
If it is within 21 days then the Z class availability is not for cheap fares as these have the advance purchase restriction, they can only be used for special contracts such as corporate customers.
Move to the red side...