Canon iR3245 Manual page 498

If you want the copies to be output in alternating directions, press [Rotate].
7
[Rotate] can be selected only in the following cases:
When you have selected Automatic Paper Selection for the Rotate mode, make sure to load only paper
that can be rotated. Otherwise, the output will not be rotated.
If there is no paper which can be rotated, the output will be collated.
7-30 Collating Copies into Sets in Page Order (Collate)
When the available paper size is selected manually
When the available paper size is loaded in both the horizontal and vertical directions
The selected paper source must have Drawer Eligibility For APS/ADS in Common Settings (from the
Additional Functions screen) set to 'On'. (See the Reference Guide.)