Activity sharing and access
Key Takeaway
With the Private organization-wide default, users with access to the records a task or event is linked to get read-only access to it. With Controlled by Parent, edit access to both linked records also lets them edit or delete it.
Activity sharing controls who can see and change tasks and events. Access does not come from one setting. It comes from who the activity is assigned to and from the role hierarchy. It also comes from user permissions and from the user's access to the records the activity is linked to.
How activities link to other records
Each task and event links to other records through two fields. The Name (Who) field holds the Who record, which is a contact or lead. The Related To (What) field holds the What record, such as an account, opportunity, or contract.
These linked records are the activity's parent records. A user's access to the parent records drives much of their access to the activity.