What is the .net framework Reference Assemblies Fa

问题:

I noticed there is a Facades folder within each of the .NETFramework Reference Assemblies folder. Starting from v4.5. All the assemblies in these folders are very small, and only contain metadata. Examples:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\Facades
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\Facades
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.1\Facades

I am curious and want to know what are these used for? Thanks!