“ProStructures Plug-in for Bentley Products” is for users who have older version of MicroStation but want to see ProSteel/ProStructures objects in MicroStation.
“ProStructures Plug-in for Bentley Products” updates “Bentley – Autodesk RealDWG” component. It is a ProStructures Object Enabler for MicroStation and suites as well for other Bentley powerplatform programs those uses “Bentley – Autodesk RealDWG” component: Bentley i-model Composer, Bentley Navigator, Bentley View etc.