How many expansions will I have to buy ?

[Airget]

It's been answered here http://forum.square-enix.com/ffxiv/t...=1#post2836394

I play on both Windows and PS4. What do I need to purchase?
You will need to purchase both the Windows and PS4 versions of the expansion to play the new content on both platforms. If you would like the exclusive bonus items from the Collector's Edition, you can get them with either the Windows or PS4 version as they will be linked to a single service account. Heavensward must be installed on the client to access the areas that will be added in Heavensward. If you are planning to log back in on a client that doesn't have Heavensward installed, make sure to log out in an FFXIV: ARR area.

[Reynhart]

It's been answered here http://forum.square-enix.com/ffxiv/t...=1#post2836394

I've pasted this exact answer and it's still not precise enough.

They specifically mentioned how you need to log out in an ARR: Area. It's obvious that if you don't have the Heavensward files installed, you wouldn't be able to log back in a new zone. But, same question, if I log out from PS4 as a Dark Knight in Ul'Dah, can I log back in PC if I haven't registered the expansion ?

This sounds much like how I went about playing FFXI on 360 and PC... I'd buy the expansion disc and code for 360, and use a friend's PC version disc to install the content on my PC.

I'd like to see it that way, but in FFXI, you didn't need two codes to play on both platforms, whereas 2.0 required that. So...still confusing.