Yes: instances of this ontology are automatically managed by the system. A user will not be able to create an instance of the ontology, or delete one.
Example: in a client onboarding case, it could make sense to have 'Contract' as a managed instance, so that users can’t remove the contract instance, because it doesn’t make sense to have zero contracts.
No: the user can add and delete instances of the ontology. Example: user can add as many Signatories as desired.