This can either be a “guesstimate” or gut feel, based on the modeller’s knowledge of how the economy functions or, alternatively, a well-articulated mathematical representation of the interrelationships in the economy in the form of an econometric model.
There are significant differences between models, depending on the different views on how the economy is seen to function, and also the use to which the model is being put. Although the various types of models are structured to capture and replicate the economy by providing for the key channels of the transmission mechanism, the emphasis tends to shift between models.