The is a required framework that allows computers to run Microsoft Office-based solutions created with Visual Studio. The x64 version is specifically designed for 64-bit operating systems to support the integration and execution of custom Office add-ins and document-level customizations. Overview of VSTO Runtime
The x64 runtime also ensures that all registry access is redirected to the , preventing conflicts with 32-bit add-ins that might run under WOW64. microsoft visual studio 2010 tools for office runtime -x64-