User: Sharing
    Sets to what degree other users can interact with you.
     
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    Note You can change any user's details in the User section of Administrative (if you have access rights)
     
Top   I grant access to my diary
  If this is ticked then other users can add appointments into your diary for you.

If this is unticked then only you can add appointments into your diary.

In either case your diary is visible to all users.