This is where I get lost. The OPC server is a standard PC server running only this software and it's completely separate from the multiple PLC's that it communicates with. The OPC server only has the minimal step7 software need to configure it which is what you get when you install it from the simatic net cd.
Then there are say five different S7 projects (one for each PLC). These comes from different vendors. The OPC server is not a part of these projects and the OPC server is not configured in any of these projects.
Do you mean that if I had one PLC with one OPC server I could/should put the OPC server inside the PLC project and configure both the PLC and OPC server from there?
Or do you mean that I could add the OPC server (a PC station?) to each of the five projects and then in each project I could download to the OPC server and the symbols from that project would automatically be transferred to the OPC?
Here is a Siemens article which describes how to add a PC station to an existing PLC project:
https://support.industry.siemens.com/cs/ww/en/view/17413251
But the article misses the relevant point: adding the connection between the PLC and the PC station (specified connection).
I know your problem with different vendors you have to include in your OPC server. I have similar projects, where I use therefore only absolute addresses instead of symbols.
I have searched for a simple solution for this with symbols, but there are always some disadvantages.
One solution is, that you can setup a Step7 multiproject. I don't know if you are familiar with this. With a Step7 multiproject you can combine several single projects to one multiproject, and add communications between the single projects. For your example, when you are the maintainer of the multiproject, you collect all single projects of the other vendors, and combine them to a multiproject. You can add your own project including the PC-station with the OPC server. Then you can synchronize the subnets from the single projects, so you can setup your connections from your PC-station including the OPC server to the single project PLCs. The main disadvantage is, that the symbolic mode is only possible with a specified connection. Means, that you have to download the connection information to the PLCs you want to communicate with. On a S7-400 you can do this in Run-Mode, most S7-300 needs a reboot.
If your OPC server is working as expected, you can "outsource" the single projects from your multiproject, and give them back to your vendors to work with. If they make a change in an address, they give you the project back, you integrate it back into the multiproject, generate system data and load your OPC server with the new informations.
It sounds a little bit complicated, but once you have setup the multiproject and know how to handle this, it's not a big deal. It's mainly used in PCS7 projects, where you may have to combine many PLCs and PC-stations.
That's the way you have to handle this, if you want to stay in the "Siemens universe".
But there are other vendors of OPC servers, which could readout the symbols from a Step7-Project.
Writing the symbol file for the Simatic Net OPC server was no solutions for me, because it makes no difference if I have to maintain the addresses inside the OPC server symbolic file, or inside the OPC client project.