User blog comment:Omega natsu2/Chapter 370: Demons Reincarnated, Predictions./@comment-42.60.151.15-20140131104028/@comment-7418318-20140131104751

That was never really established, for any of the different kinds. Some are just "storage" other's are for communication, others are to put enchantments on whatever they are installed in and some explode by means never explained, yet others are for focussing magical streams (like inside Jupiter) or to gather and release magic power (like in Nirvana)...

And none of them were ever really explained.