Configure the printer (via the Settings | Paper Handling page on the front-panel menus, or Embedded Web Server equivalent) so that each tray is unique in terms of the associated:
- Paper Size (e.g. A4 or Letter) - this should be detected automatically for the cassette trays (via sensors attached to the width and length guides in the trays), but not for the drop-down multi-purpose tray.
- Paper Type (e.g. Plain or Letterhead) - choose appropriate values from the drop-down menu.
Then when you come to print a document, select the required tray via the unique combination of paper size and type in the Printing preferences dialogue; the actual tray source value should be set to "printer auto-select", rather than attemping to select a particular tray via its identifier number.
if two (or more) trays have the same set of size/type attributes (which is likely to be the case if you haven't performed any of the above configuration), then the highest numbered tray is usually selected first, until it is empty, then any other tray with the same attributes will be used, and so on.