It is to be noted that in this text Aquinas uses the term esse to signify actus essendi. This meaning of esse goes beyond the mere fact of existing. This meaning of esse refers to something deeper: it refers to a metaphysical principle, namely, to the metaphysical principle of actus essendi.
After clarifying that from the side of the actus essendi every existing thing is said to be ens and that from the side of the essence every existing thing is said to be res, Aquinas now examines the relationship that exists between ens and quod est.
An existing thing is both ‘that which is’ (quod est) and a thing possessing the ‘act of being’ (ens.)
The point is this: the term ens does not take its meaning from ‘that which is,’ that is to say, from the quod est. The term ens takes its meaning from the metaphysical principle of actus essendi.
Just as laudans (laudantis) is the present active participle for the Latin verb laudare (laudo, laudare, laudavi, laudatus),’ ens (entis) is the present active participle of the Latin verb esse (sum, esse, fui, futurus.)
The term ens (entis) indeed is at times taken to signify ‘that which, in any way whatsoever, is,’ but in the present context Aquinas clearly puts aside this aspect of the meaning of ens to stress the direct relationship that exists between ens and actus essendi.