If you register the paper size and type on the screen displayed when you insert the paper cassette, the printer informs you when the registered information and print settings differ. This prevents you from wasting paper and ink by making sure you are not printing on the wrong paper size or printing in the wrong color due to using settings that do not match the paper type.
If displayed paper size and type differ from the loaded paper, select the item to change. If the settings match the loaded paper, close the screen.
You can also display the paper size and paper type settings screen by pressing the 
button on the control panel.
You can register frequently used user-defined sizes in Settings > General Settings > Printer Settings > Paper Source Settings > User-Defined Paper Size List.
To select a user-defined size as the paper size, set Paper Size Auto Detect to Off.