Modal Interchange Mastery Hub: The Industry Foundation Pract

In the context of modal interchange, what distinguishes a "borrowed chord" from a chord that is simply part of a parallel major/minor relationship?

When analyzing a progression like I-IV-V-I in C major, and then encountering a bVI chord (Ab major), what is the *most sophisticated* interpretation of the origin and function of this bVI chord in the context of modal interchange, assuming it resolves to V?

Consider a jazz improvisation over a ii-V-I progression in the key of D major. If the improviser frequently incorporates the melodic minor scale of the *parallel* minor key (D minor melodic minor) over the V chord (A7), what is the underlying modal interchange concept at play, and what is the primary harmonic effect achieved?