Microsoft making its own SIM card
Microsoft has discreetly released a free Windows 10 app called Cellular Data, the app has been created to allow devices to connect to the internet without Wi-Fi and it looks to use multiple national mobile networks instead of forcing you to sign up for one. The Cellular Data app will allow you to purchase mobile data plans on Windows 10 devices using your Microsoft account details. According to the app listing the Windows 10 devices requires a Microsoft SIM card, which as you there is no such thing.
Microsoft has made no official announcement regarding the Microsoft SIM but as pointed out by The Verge Microsoft may be following in the footsteps of the Apple SIM, which is a virtual SIM that allows the user to sign up with various networks.