Vijeo Citect SCADA is configured as OPC DA server and the Client is configured as OPC DA client, then is a 3rd party software like Matrikon OPC required.
Whenever there is OPC Server and client communication need to be establish which resides on two different computer, you will have to do Distrubuted COM (DCOM) settings in Server and client PC in order to get the communication done.
DCOM allow the accessibility of OPC Server and Client after passing several Operating system security and permission criteria. This setting need to be done in several Operating system services and User Account Permission.
To make above process simpler OPC Tunneller is being introduce. This is a tool responsible to create OPC server Client link between two computer without doing any DCOM settings in the PC. Thus user find it more simple and user friendly that doing DCOM setting in PC's.
So looking at the above use of OPC tunneller is not essential to get the communication done between OPC client and Server but it is option tool which enables simple and faster way to setup the link between two OPC services. Without using OPC tunneller you can do the communication done by implementing DCOM settings.