The way Square Enix handles the service is weird.
There are two distinctions: Service and Platform.
All users need to buy service to play that game. That's the monthy fee/timecard.
In order to access the game, you need to buy it for that platform and register it to your service account.
So If you had a service account and bought the game for both Mac and PC, you could hop between them. If you only have the Mac version, it'll only let you login on other Macs. The same as if you were mixing with PS3 and PS4. SE shows this with a little image now, when you look at your MogStation account. (Because someone might only have PS3 access for 2.0 and PC access for everything).
There are two distinctions: Service and Platform.
All users need to buy service to play that game. That's the monthy fee/timecard.
In order to access the game, you need to buy it for that platform and register it to your service account.
So If you had a service account and bought the game for both Mac and PC, you could hop between them. If you only have the Mac version, it'll only let you login on other Macs. The same as if you were mixing with PS3 and PS4. SE shows this with a little image now, when you look at your MogStation account. (Because someone might only have PS3 access for 2.0 and PC access for everything).