For purposes of Platform Friends, Store Purchases, and Silver Wallet, it considers you to be on Xbox. If you have a Gold subscription (or ultimate? not sure which one does it), it also applies the discount to Silver.

For the purposes of Crucible Matchmaking , it considers you to be on PC.

Behold, it shows the Windows logo in roster but considers me as signed in on Xbox as my platform

It is otherwise identical to Steam D2, down to using the same settings configuration (stored serverside, obviously), having text chat, etc.