The appropriate level of modelling detail in steady state flowsheet simulations is an important question for the calculation of thermal separation operations such as absorption, distillation, and liquid/liquid extraction. The generation of consistent mass and energy balances may often be performed employing standard implementations for the various unit operations. Surprisingly, operability and sensitivity aspects of the process may be estimated with the help of black box mass balance cosiderations. As soon as flowsheet simulation is to serve as a basis for the rating of existing or the design of new distillation units, the level of modelling detail has to be increased significantly and a Complex Topology Simulation (CTS) is required. Thus, the level of the modelling detail depends strongly on the current project state as well as the information desired. These aspects will be discussed through four examples of practical relevance concerning distillation feed sections and liquid distributors, reboiler section, condensation stage and operability aspects.