Appeon plug-ins is required for Appeon web application, Appeon uses the standard COM technology. Appeon for PowerBuilder contains some client-side ActiveX plug-ins. 

Just like other popular development software, C++ apps also use some base classes, such as MFC, ATL, etc., or other supporting files and PowerBuilder apps use PBVM, etc. Appeon Active X is similar to the PowerBuilder Virtual Machine (PBVM) except that they are implemented in JavaScript and C++, rendering UI and handling some non-database-related business logic. 
It’s just different in how it is deployed: the related EXE or other supporting files of the app will be installed in the PC’s disk in C/S structure while Appeon uses Micorsoft Active Template Library technology to install the supporting files of the app via COM objects and then demos the PB app logic in downloaded WYSIWYG Web pages.
Appeon plug-in has been digitally signed by verisign (note verisign’s sign and authentication business is actually not taken over by Symantec so in a way Appeon’s security is verified by Symantec)