' Substantial form ' (in the sense of any given substantial form) still refers to matter (and, because of this, it might not have a proper definition), while just ' form ' doesn't do so anymore (and so can be properly defined).
However, ' substantial form ' also refers to form. And form is the primary instance of Being (that is, Being in the ' Natura-view ' ). Through this substantial form we arrive, following the structure of reference, at the ' Creatura-view ', or, equivalently the domain of THEOS (the domain of the divine), that is, the Separate Forms.
We just said " ' substantial form ' still refers to matter, while just ' form ' doesn't do so anymore ". But form already is the quod quid erat esse ( = essence) (Also the forma totius [ = substantial form + non-designated matter] can be viewed as the quod quid erat esse, and also this forma totius doesn't need to mention anyhing extrinsic in its definition).
If the World only contains material beings, and can only contain material beings, then all forms (other than accidental forms) are at the same time substantial forms, that is, formed matter. This formed matter (with reference to a given thing) might be definable (that is, the term signifying it, might be definable) provided we do not consider the particular. And also then ' substantial form ' still refers in its definition to matter, but the latter is then not extrinsic anymore. In such a world form (alone) does not exist. At most it is transcendent.