If you were hoping that Call of Duty: Infinite Warfare [official website] or Modern Warfare Remastered would have the cross-platform play that some recent PC games have had, you're in for a bit of crushing disappointment. It's not even unified on PC. People who buy IW or MWR for Windows 10 through Microsoft's Windows Store are unable to play multiplayer with players who own it on Steam, Activision Support confirm. That's a bit of a bum deal.
"You can only play these titles with other users of Windows 10 on Windows Store," Activision say. Seeing as the majority of PC players will purchase or activate their copies through Steam, those that purchase them through the Windows Store may find themselves with a very bleak multiplayer experience. And no, it doesn't support cross-platform play with Xbone to make up for that.
Microsoft, asked about this by Windows Central, said "We support cross-play between devices and platforms for partners who want to enable it." That would suggest a decision at Activision is behind this, though we could only guess at their reasons. It seems unlikely that they'd release a substandard version without cause, though maybe it would've been better to simply not release it there.
If you're planning on purchasing Call of Duty: Infinite Warfare, you might want to consider these new developments before taking the plunge. You might need to end up purchasing the game on an alternate platform if you had very involved PC plans, and will be hopping mad. Rightly so. This probably isn't the greatest development that could have happened right around launch day, is it?