What are .NET Platform Extensions on docs.microsoft.com?

You can use Windows Compatibility Pack, if you want to target windows platform only as start of porting process and gradually use platform independent alternative from .NET core OR .NET Standard.

Refer for more detail: https://docs.microsoft.com/en-us/dotnet/core/porting/windows-compat-pack

The Windows Compatibility Pack is provided via the NuGet Package Microsoft.Windows.Compatibility and can be referenced from projects targeting .NET Core or .NET Standard.

Please refer NuGet link for more detail: https://www.nuget.org/packages/Microsoft.Windows.Compatibility and you can use it by executing below command:

Install-Package Microsoft.Windows.Compatibility -Version 3.1.0

It represents APIs that are shipped as NuGet packages instead of being part of the shared framework for .NET Core.

We don't have the ability right now to identify the different packages and which target frameworks they support on docs.microsoft.com, but it's something that is in our backlog to solve. So adding that as a new target enabled us to add docs for the new APIs.