One of the most successful applications of multiple input multiple output control is the control of thickness in sheet forming processes including paper making and plastic film casting. While there are many similarities between film manufacture and other sheet forming processes, the physics differs from process to process. The choice of both process model and optimal control algorithm must reflect these differences. Here we consider how the basic physics of film manufacture a reflected in two classes of process models: phenomenological (interaction) models used in conventional gauge control algorithms and mechanistic models based on engineering science. Mechanistic models are more mathematically robust than purely statistical phenomenological models for some aspects of gauge control and may prove advantageous for other sheet forming applications.