One of the most important characteristic of the spin crossover complexes [1] is represented by the bistable character at molecular level making these materials appealing candidate in switching devices, signal amplification and information storage but also as efficient contrast agents. Recently, spin crossover particles organized in a nanostructure have been produced [2] to allow systematic studies of particle size effects. These nanostructures are of considerable technological interest as full optical memories. Spin crossover (SC) compounds possess promising multifunctional properties due to their response to different external stimuli. We briefly recall that the electrons can populate either the low spin state (LS) or the high spin state (HS) and during a SC transition the solid undergoes both electronic and structural modifications, leading to important volume change (HS bigger than LS), and drastic changes in magnetic properties (HS, LS are paramagnetic and diamagnetic, respectively), in color and in dielectric constant. The perturbation which triggers the spin state can be a variation of pressure or temperature or a magnetic field. This transition may be found to be perfectly reversible or with hysteresis depending on the interactions. The response of the system can be monitored through the fraction of molecules in the HS state. Beside fundamental interest these materials promise applications as sensors or recording and displaying devices. We developed an extensive investigation of the hysteresis properties of SC compounds, using as a guideline the efficient concepts developed for the magnetic hysteresis: Preisach models, FORC diagrams [3]. After investigating the thermal hysteresis (TH) properties of the entropy-driven and light-induced transitions, we recently turned to the pressure hysteresis (PH) properties of the SC compound [Fe(PM-BiA)2(NCS)2] [4]. In this paper, we extended our first pressure measurements on more complex processes like FORCs [5, 6]. We shall discuss the differences observed between the FORC diagrams measured on increasing and decreasing branches, respectively, presenting a mechanism based on nonlinear dependences between temperature and pressure. Also, we shall present the procedure for converting the diagrams from pressure to physical parameters in the framework of the Ising-like model and we shall compare the results with the physical distribution obtained from thermal diagram. As the TH and PH are different responses of the same system, one should obtain the same physical parameter distribution for both processes and in the full paper we will present the main differences between the distributions and several hypotheses to correct the model we had used to obtain the expected results.