Side-by-side Assemblies are a Microsoft Windows solution that reduces versioning conflicts in Windows-client applications.
Side-By-Side (SxS) assemblies allow application developers to isolate the DLL resources used by their application and avoid "DLL-Hell" and other COM compatibility issues caused by multiple applications using different versions of dependency DLL files.
With Windows, application developers can build isolated applications that are fully self-describing and unaffected by changes to the registry, other applications, or other versions of assemblies running on the system. Application authors and administrators can use manifests to manage the sharing of side-by-side assemblies after deployment on either a global or per-application basis.
See also Microsoft Learn article: Isolated Applications and Side-by-Side Assemblies
