Reference: Configure the ISAPI connector on Windows - terms
ISAPI and CGI Restrictions ISAPI and CGI restrictions are request handlers that allow dynamic content to execute on a server. These restrictions are either CGI files (.exe) or ISAPI extensions (.dll). You can add custom ISAPI or CGI restrictions if the IIS configuration system allows this. [http://technet.microsoft.com/en-us/library/cc730912(WS.10).aspx].