<signalIL>@signalFunction is about the function of the signal as seen from the interlocking
"main" could be seen as a duplication but its meaning is more anything else but the specific uses - "The main signal is
a normal signal for train traffic protection which is neither used as block, entry, exit nor intermediate signal."

<signalIL>@isVirtual is especially considering ETCS systems were the interlocking in its data still has signals which
are switched to particular aspects. But on the outside there are only marker boards to identify the position of these
interlocking only signals. In that sense they are physical outside but virtual for the interlocking.
Whereas <signalIS>/<signalConstruction>@type="virtual" really means no physical object outside.