Master-detail field settings for reparenting and record access
Key Takeaway
On a master-detail relationship field, the "Sharing Setting" decides how much access to the master record a user needs to work with its detail records. "Allow reparenting" decides whether a detail record can move to a different master record.
A master-detail relationship ties a detail (child) record to a master (parent) record so closely that the master record controls who can see and change the detail record. Two settings on the master-detail relationship field shape this relationship: the Sharing Setting and Allow reparenting.
How a detail record depends on its master record
The master-detail relationship field is required on the page layout of detail and subdetail records, so every detail record must be linked to a master record.
Detail and subdetail records inherit their security settings and permissions from the master record, so you can't set permissions on the detail record on its own. Lookup relationships are different, because they don't support sharing or roll-up summary fields.
The Owner field isn't available on detail and subdetail records. Instead, Salesforce sets it automatically to the owner of the master record. Sharing rules, manual sharing, and queues all need the Owner field, so custom objects on the detail side can't have any of them.
Unlike a custom object on the detail side, a custom object with no master-detail relationship uses ownership and the org-wide sharing model.
For example, imagine a record on a custom object with no master-detail relationship. A user who doesn't own the record needs Public Read Only or Public Read/Write to view it. To delete the record, a user must be the owner or above the owner in the role hierarchy.