Submodeling, as the name suggests, is the modeling of a portion of a larger (global) model to capture the effects of refined mesh in a smaller model. This is done to avoid having the whole assembly meshed with fine elements.
We use coarse mesh in the global model, as the local features of the submodel do not have any impact on the overall response of the system. We then define a refined submodel with displacement and temperature results applied to the submodel. The most common utility of this approach is to refine a critical region or region of interest without solving for the whole model. Another important aspect is to study small changes like adding a fillet. With local design changes come the parametrization of the design, which is much easier in the smaller submodel. Apart from this, we can model localized nonlinearity that might not have been captured in the global model.