The higher-level modules in this top-down method are evaluated first, followed by the lower-level modules. The modules are then appropriately integrated. Lower-level modules here relate to submodules, while higher-level modules refer to the main module. This method makes use of Stubs, which are mostly used to emulate the invoked submodule; if it is not developed, the Stub serves as a temporary replacement.   The lower-level modules in this Bottom Up technique are tested first, followed by the higher-level modules. The modules are then appropriately integrated. Higher-level modules here relate to core modules, whereas lower-level modules refer to submodules. This method makes use of test drivers, which are primarily employed to initiate and pass necessary data from upper-level modules to lower-level modules as necessary.