Lookup parent-delete field behaviors
Key Takeaway
With an optional lookup field, you choose what happens to child records when the parent record is deleted: clear the field, block the deletion, or delete the child records too.
When a record on the parent side of a lookup relationship is deleted, Salesforce has to know what to do with the child records that point to it. For an optional lookup field, the app builder decides by picking a delete behavior when creating the field.
Lookup and master-detail relationships
A lookup relationship links two objects so users can see related data, but it is looser than a master-detail relationship. You can see the difference when a record is deleted. Deleting a master record always deletes its detail and subdetail records. With a lookup relationship, you choose what happens to child records when the parent record (also called the lookup record) is deleted.