By default, Simile uses the same time step to update all the model state variables. However, if you are modelling a system containing trees and crops, then you might very well want to model the trees on an annual basis (time step of one year), and the growth of the crop on a weekly basis (time step of 1 week).
Simile enables you to specify a time step category for any submodel. For each new time step category that you request, Simile adds an extra Update every entry in the Run Control dialogue window, and that is where you specify the actual time step (e.g. 0.01) to be used for each category.