If the type of document should not acquire local roles from the
parent documents (and modules), uncheck the "Acquire Local Roles" on this portal type configuration.
If the security of sub-document is different, you usually don't want
to acquire all roles from the parent. For example, this is the case for Assignment
documents, a user can be assigned some person documents, but we don't want this
user to open assignment which are physically contained in this person.