Distinguishing A5R and STMTR Paper
When A5R or STMTR paper is loaded in the paper drawer, you can set the size to use for the operation.
1.
Press 
(Settings/Registration).
(Settings/Registration). 2.
Press [Preferences] → [Paper Settings] → [A5R/STMTR Paper Selection].
3.
Select [A5R] or [STMTR] for each paper drawer → press [OK].