Fuel cycle simulator development started many years ago by several research and engineering institutions or consulting firms for a wide range of applications. To improve confidence in the results, institutions may be tempted to increase the complexity of their software even if this complexity might not be necessary. On the other hand, some simulators may be used outside their range of validity when used in very specific applications. The FIT (Functionality Isolation Test) project is an international effort devoted to improve the confidence in the data produced by fuel cycle simulation tools. The scientific goal is to determine the optimum level of detail a fuel cycle simulator needs according to the type of study and the required confidence level. The project relies on a wide variety of fuel cycle simulators with a large range of complexity levels. The FIT project consists of isolating the impact of one targeted functionality on fuel cycle simulations. The impact of the functionality is assessed using a set of simple basic exercises specifically designed for this purpose, called ”functionality isolation.” The present work focuses on the impact on simulation results of using a fuel loading model (a relation that links the stock isotopic composition with the fresh fuel fabrication according to the reactor requirements) or a fixed fraction approach (the fresh fuel fissile fraction is fixed and does not depend on the stock isotopic composition). The paper first presents the FIT project. The exercise design is described and results show that using a fuel loading model approach has an important impact on fuel cycle outputs under certain conditions that are described. This result is reinforced by the fact that all fuel cycle simulators used in this exercise provide similar conclusions.