With the same size of paper loaded into multiple trays, the machine can use one tray for one function and another tray for another function.
For example, you may load white A4 paper into Tray 1 to use when making copies, and yellow A4 paper into Tray 2 to use when printing received faxes. You can then easily identify for which purpose paper has been output. You can perform this setting in [Ppr Tray Priority:Fax] in [Tray Paper Settings]. See "Tray Paper Settings", Connecting the Machine/System Settings.