So, I am trying to figure out how exactly we can do this and getting very more and more frustrated after spending 2hrs of research. According to SQEX:

Windows only: To play three characters simultaneously
- A Square Enix account
- A copy of the Windows version
- Three registration codes obtained from the product or the "FINAL FANTASY XIV Service Center"
*You can register up to eight registration codes per Square Enix account
I can't find any information on where to buy those registration code, on the mog station nothing points to being able to do that.

The only thing I can see is the ability to create multiple service accounts, but you have to actually buy the game again. So why bother saying :
- A copy of the Windows version
when you would actually need one copy per service account.

Can a mod please come and clarify this?

Thanks