The stable NAME of a signal: which port of which component of which instance. This is WaveIndexT without SimArrayIndex, and that omission is the point. SimArrayIndex is a step-array index handed out by one build of one simulation and means nothing outside it, which is why a selection has to be re-resolved by the other three fields whenever the simulation is rebuilt. A SignalId survives a rebuild by construction, so it is what the model should hold.