var value, mySysTag

mySysTag = "@MemoryInformation_1.Used";

value = Tags(mySysTag).Read();

Tags("MemoryUsed").Write(value);

I tried this in simulation and on a physical panel using V19 Update 1 and it works as expected. The system tag examples in the demo project are as follows and they all work.

• @MemoryInformation_1.Used
• @MemoryInformation_1.Total
• @MemoryInformation_1.UsedInPercent
• @MemoryInformation_1.AvailableInPercent

In addition, with a little digging I found that these system tags and others are defined in the following file as well as any complex types used:

C:\Program Files\Siemens\Automation\WinCCUnified\WCCIL_PLF\config\SystemData.rdf

With a little ChatGPT parsing we can get the full list of possible system tags including the hidden tags.  I suspect that some are only valid for the PC runtime and not for panels as I couldn't get anything useful from the @ProcessorInformation_1 or @DiskInformation_1 tag structure. The @Heartbeat_1 tag was working as well but I didn't check most of the others. 

I only extracted the tags that were prefixed with @ but there was a lot of reference to the HMI and OPC UA Server connections. Maybe some other magic strings work also.

Here is the list I extracted with some of the nested complex types: