I’ve never been to the Holon region myself, largely because they’ve never needed a storage system. After the incident that resulted in the “delta pokémon,” the place has been considered a sort of pokémon reserve to protect them—wherever human settlements aren’t, anyway.

The delta pokémon, meanwhile, are a fascinating subject. It’s launched plenty of scientific investigations and resulted in the reevaluation of the nature of types that is partially responsible for the fairy reclassification. To this day, the delta pokémon remain largely a mystery; no one knows for certain how the type changes happened or why. At least with the newly discovered Alola forms, that’s completely understandable due to environmental factors. The delta type shifts, meanwhile, just … happened.

Granted, yes, we know of the history that led up to that point—the search for Mew, that is, as well as the electromagnetic pulse that resulted in the change. It’s just that no one truly knows how an electromagnetic pulse triggered the mutation, especially given the fact that such pulses have never produced the same results outside of Holon. Some pokémon, such as the magnemite line, rely on high-powered electromagnetism to survive, so something doesn’t entirely add up about the whole situation. It doesn’t help that the Holon Research Tower has been inaccessible to the public and scientific communities, following investigations of possible unethical experimentation.

As for the use of delta pokémon in league fights, it’s actually illegal to remove pokémon from Holon at all unless for research purposes (due to the fact that, even over a decade later, the investigation into delta pokémon is still ongoing, and they’re by and large a finite resource), so it’s generally frowned upon to use them to battle, I’m afraid.