I think this terminology is a throw back to the early days of plasticizing PVC. Some early PVC plasticizers were actually lubricants and not plasticizers as we think of them today. They operated as process aids, and were somewhat effective in lowering compound viscosity, adding flexibility, and improving extrusion rates.
More specifically to your question, internal plasticization is sometimes used to describe PVC copolymers. That is copolymerization of vinyl acetate (internal plasticizer) w/ vinyl chloride results in lower crystallinity and lower modulus for the polymer. Vinyl acetate is sometimes called an "internal plasticizer" of PVC, in this case.
Homopolymer PVC, w/ out a comonomer would have a higher modulus, and would respond to "external" plasticizer, like DIDP for example. This latter case is the classic f-PVC method for lowering Tg, improving elongation, etc. using GP and specialty "external" plasticizers. External plasticizers are much easier to fine tune; to dial in the proper Tg, flexibility, etc., compared to copolymers route. Copolymers are less likely to exude and cause other diffusion or migration issues than external plasticizers.