Autofill settings in iTunes on PC

In the iTunes app on your PC, use Autofill settings to add songs in a certain order, set how much reserved space is required, and more.

Replace all items when Autofilling				Replaces all songs every time that you use Autofill.
Choose items randomly				Syncs a random list of songs to your device.
Choose higher rated items more often				When you select “Choose items randomly,” higher rated items are added more often than those with lower ratings.
Reserve space for disk use				Set aside space to use as a hard disk. Drag the slider to set the disk capacity.

Helpful?

iTunes User Guide