Prime matter is the principle of individuation, but only when it is informed by unterminated dimensions. Only then numerical diversity is possible.
If unterminated dimensions are considered in their distinctive nature as unterminated dimensions, they individuate their subject. If, on the other hand, they are considered as accidental form, i.e. as inhering in their subject, they are individuated, meaning that they are rendered the dimensions of this subject (and so are individuated) by that subject, like every other accident (is individuated by its subject). Those dimensions are then individual, but they are not, of course, individuals.
Prime matter under dimensive quantity-considered-in-its-pure-nature (i.e. involving unterminated dimensions) is the principle of individuation concerning the historical individual (i.e. the total of all successive system stages of a given intrinsic being).
Prime matter under dimensive quantity-considered-as-involving-terminated-dimensions is the principle of individuation concerning the here-and-now individual (semaphoront),
or, said differently :
The termination (delimitation), and thus the individuality, that goes with matter under terminated dimensions, is the effect of the individuation of the subject. This individuation of the subject took place by the information of prime matter by unterminated dimensions. This informing is part of, and is governed by, the informing of prime matter by the substantial form. All this ultimately results in terminated dimensions.
We can still explicate further :
Dimensive quantity, considered as an accident, inhering in its subject, is individuated by the unterminated dimensions through the mediacy of the subject. Dimensive quantity considered as unterminated means that the dimensions are terminated (they always exist as terminated), but, only having one determination or another, not having this determination. And the variations in the determination are set within specific limits demanded by the Essence of the thing. Unterminated dimensions so conceived is pure quantity, (and so) quantity not considered as an accident inhering in this subject. Only quantity as terminated (dimensions) is an accident, inhering in this subject.
The principle of individuation as given here, and involving terminated as well as unterminated dimensions, allows for historical individuals (as well as for here-and-now individuals) to exist : Thus the designation (delimitation) of matter (that as such figures in the principle of individuation) should not totally be fixed, but it should be fixed disjunctim (that is, in a disjunctive way), i.e. either totally determined in that direction, or totally determined in this direction, etc., depending on the given state of the thing we're looking at. Each such a state represents a here-and-now individual, while the totality of these states of the same thing represents the historical individual. And this is indeed satisfied by the above expounded principle of individuation (taken from St Thomas). When now designated matter is removed, then indeed only the particular is being abstracted (in the sense of being removed), resulting in an individual Essence, and consequently not in an extramentally existing universal, i.e. not in a universal existing in extramental Reality (which here means a universal existing in the thing).
So if we switch from Socrates (as individual [and contingent] suppositum) to homo (or Socrates-for-example), we do not create a universal, we still have an individual Essence, because, although removing matter under terminated dimensions, we still have matter under unterminated dimensions, individuating the subject.
The materia signata then is prime matter under terminated dimensions.
The materia non signata then is prime matter under unterminated dimensions.
And it is the materia non signata that still belongs to the Essence (which is the forma totius, consisting of materia non signata + substantial form).