Hi,
Yes, currently they aren’t much different than what you mentioned- expression tags, or more directly, derived tags. They are lighter weight than both of those in that they don’t create expressions in memory (though derived tags don’t need to actually create expressions if you don’t define read/write expressions outside of the default), though that’s not why they were created.
The main reason why they were created is because we want to make it really easy to drag tags from MQTT or other “managed” providers into your normal tag providers, and we thought it would be best to create a very simple/direct reference instead of a potentially more confusing expression/derived tag. ALSO- they were originally supposed to be what you first thought- complete tag symlinks that pulled in all sub properties. I think this would probably be ideal, but it became very complicated when thinking about how alarms and history would work.
So where we’re at now is something that isn’t terribly useful, but not really a problem to have, so… we probably have about 2 weeks to figure out whether we should remove it or make it better.