So, in one place, can someone define each of the following?

6.8mm SPC
6.8mm SPCII
6.8 x 43mm
6.8 ARP

As I start shopping for components to build my 6.8mm AR15, what do I need to know about the differences between these as they relate to what components to buy and what ammo I can shoot. I will be handloading and I like to beef my loads up.

Thanks!