Ubiquity implies residence of being in space, both spiritual and material. By spiritual space we mean the existence of different created spirits and nothing more.
By material space we mean the extension of matter.
That God can act on or reside in spiritual beings does not involve any difficulty.
But how can he reside in material space, space properly so called?
It is evident that a spiritual being cannot dwell in space by a contact of extension, since spiritual being is the very opposite of extension.
Therefore, a spiritual being can only dwell in space by acting on it.
The presence of the whole being of God in the action by which he creates, sustains, and acts in spiritual and material space, is ubiquity.
Immensity is the presence of the whole being of God in his action. The difference between the two lies in this: that ubiquity implies a relation to created objects, whereas immensity implies no such relation. We say, then, the presence of the whole being of God in his action, because God is pure actuality, action itself. If, therefore, in his action we did not suppose the presence of his whole being, we should establish a division in God; that is, we should suppose his being and his action to be distinct, which they are not, and this distinction would imply a development in God, which is contrary to his being action or actuality itself.
It will easily be remarked that immensity is an attribute which flows immediately from the idea of God being actuality itself. We may therefore conclude that he is immense.