@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx procCtx, Object... nodeOutputs)
        throws SemanticException {
      Operator<?> op = (Operator<?>) nd;
      AuxOpTreeSignature treeSig = pm.getAuxSignatureOf(op);
      pm.merge(op, treeSig);
      return nd;
    }
This signature is used to establish related parts to connect pre/post PPD optimizations.
ql/src/java/org/apache/hadoop/hive/ql/plan/mapper/AuxOpTreeSignature.java