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 seeing unexpected colors due to using settings that do not match the paper type.
If the displayed paper size and type differ from the loaded paper, update the settings. If the settings match the loaded paper, close the screen.
You can also display the paper size and paper type settings screen by selecting 
on the control panel.
You can register frequently used user-defined sizes in Paper Size > the User-Defined tab.