The RuntimeKey property is required only if you are using the control to print your pages, not using any of our UI components, or when using the eXPrint/COM component in the Isolated Application. 

Isolated applications are self-describing applications installed with manifests. Isolated applications can use both private assemblies and shared assemblies.

An application is considered fully isolated if all of its components are either shared side-by-side assemblies or private assemblies. It is called partially isolated if it uses some components that are not side-by-side assemblies. Note that if an application uses some components that are not side-by-side assemblies, or uses private assemblies, the application may be affected by the installation or removal of other applications on the system. For more information, see Side-by-side Assembly Sharing.

Developers are encouraged to design isolated applications and to update existing applications into isolated applications for the following reasons:

• Isolated applications are more stable and reliably updated because they are unaffected by the installation, removal, or upgrading of other applications on the system. 
• Isolated applications can be designed so that they always run using the same assembly versions with which they were built and tested. 
• Isolated applications can use functionality provided by the side-by-side assemblies made available by Microsoft. For more information, see Supported Microsoft Side-by-side Assemblies. 
• Isolated applications are not tied to the shipping schedule of their side-by-side assemblies because applications and administrators can update the configuration after deployment without having to reinstall the application. This would not apply in the case where only one version of the assembly is being made available. 
• A fully isolated application may be installed by using the xcopy command. Windows Installer can also be used to install an isolated application without impact to the registry. For more information, see Installation of Win32 Assemblies.

Shortly, the Isolated COM allows your application to use ActiveX components without having to register them.