If you sign out of your Apple ID settings on Mac
Before you sell or give away your Mac, be sure to sign out of your Apple ID in System Settings on that Mac. See Sign in or out of your Apple ID on Mac.
What happens to your iCloud data when you sign out?
When you sign out, 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 no longer keeps them up to date with changes on your other devices. If you choose not to keep copies, your documents and data stored in iCloud are removed from your Mac.
You’re also asked whether you want to finish downloading photos and videos that aren’t fully downloaded from iCloud Photos to your Mac or remove the items that have been downloaded. After you select an image, you can adjust the way it looks.
Your bookmarks and Reading List remain in Safari, but the iCloud Tabs list of web pages open on your other devices is no longer available.
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.
Copy iCloud documents and data before signing out
Before signing out of your Apple ID, you can selectively copy or archive iCloud documents and data to your Mac.
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.