But perhaps this is not entirely correct : The quid est of a property is ultimately answered, via propter quid, by indicating the cause : A is what it is, in virtue of B. So it is not only indicated that there is a cause (of the residing-in of the property), but at the same time what this cause is ( Here, B [in contrast to, say, C] ).
As regards (a given) Substance the answer to the question quid est also consists in indicating the cause, namely the Form, and so also (as in the case of a property) not (consists) only in whether there is a cause (but also what this cause is).
Let us see what these causes are.
The cause of a (necessary) property (ultimately) is the appropriate subject (substrate, carrier), and, consequently, an esse sympliciter.
The cause of (the given) Substance is the Form, and Form is not an esse simpliciter, but a principle (or cause) of the esse simpliciter.
An thus the answer to the question quid est with respect to a (necessary) property, refers to the esse simpliciter (and not to the esse secundum quid), while the answer to the question quid est with respect to (a given) Substance refers to the cause of the esse simpliciter, and so it indirectly refers to the esse simpliciter.