It allows you to edit and create directB2S backglasses for the B2S.Server using a "WYSIWYG" editor.
Documentation is available in the package as html-help but can also be read online here.
NOTE If you are having problem opening the html-help B2SBackglassDesigner.chm file, try right click and select Unblock. This is a security feature of Windows
To build either B2S.Server or B2S.Designer, open the *.sln files in Visual Studio (any version from 2019 should work) and select build->Build Solution from the menu. Microsoft has prepared VM's that include Windows 11 and Visual Studio in one bundle. The licence for these is usually 3 months. This is an easy way to test if it works without having to install lots of different components.
You can run this application on linux with wine:
export WINEPREFIX=./wine
export WINEARCH=win32
winetricks dotnet40
winetricks gdiplus
# I could not make it run with wine-mono but it was fine with dotnet
winetricks remove_mono
winetricks dotnet48
# The application can now be started with
wine B2SBackglassDesigner.exe