"Although the mind’s status as a substantial form may seem at risk because of its meager explicit textual support, Descartes suggests that the mind a “substantial form” twice in a draft of open letter to his enemy Voetius:
Yet, if the soul is recognized as merely a substantial form, while other such forms consist in the configuration and motion of parts, this very privileged status it has compared with other forms shows that its nature is quite different from theirs (AT III 503: CSMK 207-208).
Descartes then remarks “this is confirmed by the example of the soul, which is the true substantial form of man” (AT III 508: CSMK 208). Although other passages do not make this claim explicitly, they do imply (in some sense) that the mind is a substantial form. For instance, Descartes claims in a letter to Mesland dated 9 February 1645, that the soul is “substantially united” with the human body (AT IV 166: CSMK 243). This “substantial union” was a technical term amongst the scholastics denoting the union between a substantial form and matter to form a complete substance.
Surely Descartes maintains that mind and body are two substances but in what sense, if any, can they be considered incomplete? Descartes answers this question in the Fourth Replies. He argues that a substance may be complete insofar as it is a substance but incomplete insofar as it is referred to some other substance together with which it forms yet some third substance. This can be applied to mind and body as follows: the mind insofar as it is a thinking thing is a complete substance, while the body insofar as it is an extended thing is a complete substance, but each taken individually is only an incomplete human being...
This account is repeated in the following excerpt from a letter to Regius dated December 1641:
For there you said that the body and the soul, in relation to the whole human being, are incomplete substances; and it follows from their being incomplete that what
they constitute is a being through itself (that is, an ens per se; AT III 460: CSMK 200).
The technical sense of the term “being through itself” was intended to capture the fact that human beings do not require any other creature but only God’s concurrence to exist. Accordingly, a being through itself, or ens per se, is a substance. Also notice that the claim in the letter to Regius that two incomplete substances together constitute a being through itself is reminiscent of Descartes’ remarks in the Fourth Replies. This affinity between the two texts indicates that the union of mind and body results in one complete substance or being through itself. This just means that mind and body are the metaphysical parts (mind and body are incomplete substances in this respect) that constitute one, whole human being, which is a complete substance in its own right. Hence, a human being is not the result of two substances causally interacting by means of contact and motion, as Gassendi and Elizabeth supposed, but rather they bear a relation of act and potency that results in one, whole and complete substantial human being."
https://iep.utm.edu/rene-descartes-mind-body-distinction-dualism/