It's the 6mmx39 and 6mmPPC that are essentially the same. The 6x39 has large primer pockets whereas the 6mmPPC has small primer pockets. There are other minor differences too.
As others stated 6x45, 6mm/223, and 6mmTCU are very similar
Of course, you can make your 6mmx39cases from Remington 7.62x39 brass, and then have small primer pockets, too................................