A linear system is when a response y (e.g. oil supply) can be modeled as a linear function of a set of independent predictors x (e.g. price, demand, GDP growth) using usually a generalized least square approach. Interactions between predictors and feedback from the response to predictors make it impossible to use that simple approach. When a system is stable, you can assume that some predictors are known and set them to a fixed value with a good confidence (e.g. demand growth humming at a constant 2% per year).