Grey Box Model is specify the mathematical structure of the model explicitly, including couplings between parameters. It is useful when know the relationships between variables, constraints on model behavior, or explicit equations representing system dynamics. Almost all models are grey box models as opposed to black box where no model form is assumed or white box models that are purely theoretical. Some models assume a special form such as a linear regression or neural network. The model can be deterministic or stochastic depending on its planned use.