It happened that for historical reasons we were taking the runTag for a restG4 simulation from the title of TRestG4Metadata section.
This was interesting in the case when we were defining several TRestG4Metadata sections so each section would define its own runTag. But now the trend is that one RML contains only one TRestG4Metadata definition.
Now, the runTag will be taken from <parameter name="runTag" value="xxx" /> defined inside TRestRun.
If no runTag or runTag="" then we still continue retrieving the runTag from the TRestG4Metadata title.