The distinctions of charity are parallel to those of grace. Charity is infused at the moment of the justification which it effects. But its own effects on the operative faculties can be augmented and, in this case, it is possible to talk of 'acquired' charity. The charity which is infused into the soul has to be created, but the full effect of justification cannot be explained unless in some sense God's uncreated charity is communicated to us, so that the distinction became necessary to explain the metaphysics of justification.
Whether grace achieves in us a 'substantial' or an 'accidental' modification was a question bound to arise in any discussion of the modalities of justification inside Aristotelian categories.