What is a difference between SIMPLIS and SIMMETRIX models in MPSmart

What is a difference between Simmerix and Simplis model and when each one should be used?

SIMetrix is a SPICE based tool for general analog circuit simulation, while SIMPLIS is a specific tool for the fast power supply simulation by modelling devices using a series of straightline segments rather than solving non-linear equations as SPICE does and all the MPS ICs models are created using SIMPLIS. However, MPS does offer SIMetrix models such as MPQ4481 on the website. The model type usually depends on the request.