Windows Identity Foundation introduces a new ClaimTypes class. It contains predefined claim type URIs for claims defined by OASIS and Microsoft. In the WIF SDK project templates for a custom STS this ClaimTypes class is mixed with the one already in System.IdentityModel.Claims, which is rather confusing. So, what's the difference?

Functionally: None. All claim type URIs in Microsoft.IdentityModel.Claims.ClaimTypes are identical to corresponding types in System.IdentityModel.Claims.ClaimTypes. That said, Microsoft.IdentityModel.Claims.ClaimTypes adds a few new claim types.

Technically: Claim types in System.IdentityModel.Claims.ClaimTypes are defined as static read only string properties, whereas in Microsoft.IdentityModel.Claims.ClaimTypes the claim types are string constants.

My advice: for clarity always use Microsoft.IdentityModel.Claims.ClaimTypes.