The crop models in MCYFS where originally developed to simulate crop yield for a single location (or region) where weather, soil and crop characteristics are assumed homogeneous. To apply the crop models on a large scale the models have to be run for every location with a unique set of weather, soil and crop characteristics. The weather and spatial and temporal crop characteristics are assumed to be homogeneous per grid cell. The soil characteristics are assumed to be homogeneous per . In other words: the crop models have to be run for every unique combination of grid cell and STU. These combinations are called simulation units.
To be able to aggregate model output to administrative regions it is necessary to know the relation between simulation units and administrative regions. For this purpose 's are created which results from intersecting the weather map (25x25 km grid cell), soil map ('s), and administrative regions map ( level 3).