To be and that which is are different. For to be itself is not yet, but that which is, having received a form of being, is and subsists. Vivere est esse viventibus . For a living thing, to be is to live. Esse substantiale .
Angels are subsistent forms.
They are kinds of existent, and their actuality is measured by and characterized by their essence or nature. But existence is not of their nature. They are not necessary existents in that sense. The angelic essence is as potency to the act of existence. It remains always dependent on the divine causality.