That 30 days free sub from ARR pack is that what you mean?
Are you on trial version now?

1) You register as a free trial then you can play 14days for free lvl is only up to 20, you can create few characters to any server.
2) You purchase FF14 ARR, that package are include 30 days free sub, you can use that serial number to upgrade your current account or creating new one.
3) If you buy another sub time then it will be added to your account, free 30 days from ARR + additional sub time.

That's from my understanding.