Apple Motion 4 - Manual (Page 254)

While Motion makes an effort to correctly interpret how objects you add to your project
should appear, sometimes it isn’t possible to correctly identify how a source media file
should be treated. Make sure that each object you add to your project has the correct
settings in its Media tab so you can successfully mix and match objects with different
pixel aspect ratios, alpha channel types, and frame rates.
Because Motion is a nondestructive application, changes made to these parameters are
not applied to the source media files on disk. They simply affect how objects are drawn
within Motion.
254 Chapter 6 Creating and Managing Projects