COM/DCOM is the underlying technology provided by Microsoft Windows, that allows applications to communicate and share data with each other. COM/DCOM was created to replace the legacy DDE technology.
Unlike DDE, COM/DCOM is a securable technology. Each COM object (an OPC Server or Client in this case) can be secured such that an administrator can specify which users and/or user-groups can access which object, and what level of access they have. This is a great feature.
Each COM object must run with a login account, in the exact same way that you (the user) must log onto Windows. The principle is the same.
Custom/Inherited Security
Security issues can arise when an OPC Client is refused access by the OPC Server (this is actually managed by the Operating System, not the software itself).
The OPC Web Client engine provides you with the flexibility to let the software use its own custom security settings, which in many cases will bypass many of the security errors that users have become accustomed to.
Alternatively, if you disable the custom security then the engine will inherit its login credentials from the DCOM configuration utility within Windows.
Conclusion
The OPC Web Client by default uses Custom Security which will in most cases prevent you from having to configure DCOM security settings. In some cases where you need to specify your own security settings, you will have that capability.
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