The former two can be thought of as more opportunity driven, while the third is more complex as this could be both due to either opportunity or necessity.
Ystyriwyd bod hyn yn angenrheidiol gan fod y defnydd o'r pŵer yn benagored a gellid ei ddefnyddio i fod yn gymwys i 'amgylchiadau eraill o angen eithriadol' anhysbys neu heb eu diffinio.