In macOS Mojave or earlier, open iTunes, choose iTunes Preferences from the menu bar, then click Advanced.

When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in iCloud automatically.

Each file stays right where you last saved it, and downloads when you open it.

Each segment of the bar is an estimate of the storage space used by a category of files.

If some recommendations are already turned on, you will see fewer recommendations.

When storage space is needed, only the files you recently opened are kept on your Mac, so that you can easily work offline.

Files stored only in iCloud show a download icon, which you can double-click to download the original file.

When storage space is needed, only space-saving (optimized) versions of photos are kept on your Mac.

When storage space is needed, only the messages and attachments you recently opened are kept on your Mac.

When storage space is needed, movies or TV shows that you purchased from the iTunes Store and already watched are removed from your Mac.

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Click the download icon next to a movie or TV show to download it again.

Mail automatically downloads only the attachments that you recently received.

You can manually download any attachments at any time by opening the email or attachment, or saving the attachment to your Mac.

Mail downloads an attachment only when you open the email or attachment, or save the attachment to your Mac.

Click the Review Files button, then choose any of the file categories in the sidebar, such as Applications, Documents, Music Creation, or Trash.

Other categories show the total storage space used by the files in each app.

You can then open the app and decide whether to delete files from within it.

You can also control those settings directly within each app.

Then click Options next to iCloud Drive: Store in iCloud turns on the Desktop Documents Folders setting.

In macOS Mojave or earlier, choose Apple menu System Preferences, click iCloud, then click Options next to iCloud Drive.

Store in iCloud turns on the Desktop Documents Folders and Optimize Mac Storage settings.

Store in iCloud selects iCloud Photos and Optimize Mac Storage.

Optimize Storage selects Automatically delete watched movies and TV shows.

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