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.

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iTunes User Guide