Spectre noise type definitions - Custom IC Design - Cadence Technology Forums - Cadence Community
To my understanding, using on/off option for the instances divides them into two categories (on category and off category). Noiseon_type determines the noise sources for the on category and noiseoff_type determines the noise sources for the off category. To cover more combinations and have a better understanding of these options, you can create some cases where one of the inverters is in the off category.