If you sign out of your Apple ID preferences on Mac
Before you sell or give away your Mac, be sure to sign out of your Apple ID on that Mac. When you do, your documents and data stored in iCloud are removed from your Mac. Your bookmarks and Reading List remain in Safari, but the iCloud Tabs list of webpages open on your other devices is no longer available.
You’re asked whether you want to keep copies on your Mac of documents stored in iCloud Drive and data stored in iCloud by Contacts, Calendar, Reminders, and Messages. If you keep copies, iCloud doesn’t keep them up to date with changes on your other devices.
You’re also asked whether you want to download or remove photos and videos that aren’t fully downloaded from iCloud Photos to your Mac. After you sign out of iCloud in iCloud preferences, downloaded Photos and videos aren’t kept up to date with changes on your other devices.
Copy iCloud documents and data before signing out
Before signing out of iCloud, you can selectively copy or archive iCloud documents and data to your Mac.
iCloud Drive: For instructions on copying documents to your Mac, see Use iCloud Drive to store documents.
Mail: For instructions, see Move or copy messages.
Contacts: For instructions, see Export and archive contacts.
Calendar: For instructions, see Import or export calendars.
Documents and data you copy to your Mac aren’t kept up to date with changes on your other devices.
After signing out of your Apple ID on your Mac, you can still access your iCloud documents, data, photos, and videos on your other devices set up using your Apple ID, and on iCloud.com.
If you sign in with your Apple ID again on your Mac, your iCloud documents and data reappear on your Mac. Photos and videos you haven’t yet downloaded from iCloud Photos also appear on your Mac.