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it is probably just the purpose of the layer you are using for the label, to find out which layerurpose pair to use, open the lvs rules ans identify the layer map section (it could be in a separate included file), in that section you will see the stream layer/purpose paur mapped to a name used for stamping metals and occasionally poly, diffusions and wells directly
I think in TSMC it is usually the 'pin' purpose that is used for stamping