For pt.I see ibid., vol.3, p.2545-59 (1991). The additional boundary condition (ABC) is considered from the point of view of the angular momentum conservation law. This ABC is shown to be representable as a continuity condition for the macroscopic flux of internal angular momentum and is proved to be consistent with the ABC used previously by various authors, which reveal the physical reasons that lie behind the very existence of the ABC problem in macroscopic crystal optics.