If you plan on buying Call of Duty: Infinite Warfare and jumping into multiplayer with your friends, don't buy it from the Windows 10 Store. In a FAQ on publisher Activision's web site, the company claims that PC gamers won't be able to play with those who buy from other stores. The same goes for Call of Duty: Modern Warfare Remastered.
"Can I play Call of Duty: Infinite Warfare or Call of Duty: Modern Warfare Remastered for Windows 10 on Windows Store with my friends that are playing on Xbox One?" the FAQ asks. The response:
So if your friends — or even online strangers — bought from Steam, a digital code from Amazon or GameStop or anywhere else, you won't be playing with them, and that will likely be a majority of gamers with the title. You'll be sandboxed in with others who purchased through the Windows 10 Store.
If multiplayer is at all important to you, buy the game literally anywhere else. We've reached out to Activision and Microsoft for more information and will update this post if we get a response.