Master detail relationship field can’t be created directly if record already exists. Instead we have to create a lookup field first then fill all the records with that lookup field. After that we can convert lookup field to master – detail relationship. Deletion of master record will also delete detail (child) records Where as in lookup relationship if we delete any object then other object is not deleted.
Custom reports on activity or opportunity works on role hierarchy. In standard report types for activities and opportunities one can bring additional users in who aren’t part of the role hierarchy selected. But with custom report types for activities and opportunities this cannot be done. Users who are not assigned to any role can not be able to view custom activity or opportunity report data.