It is not a proper world in its own right, and was only split off from World 12B (our own world) by accident. It resembles World 12B a great deal, with only one significant difference, which would not be the case if it were a properly separate world.
The significant difference is that World 12Ba has an extraordinarily high proportion of people with magical ability. The proportion is even higher than is generally realised, because "witches" are systematically persecuted - and liable for death by burning in many places - so most people keep their abilities carefully hidden.
For details of how World 12Ba came into being, see Witch Week (spoilers).