Assumption preferences, set with Preferences, and then Assumptions, are similar to those for forecasts, described in Setting Forecast Preferences. By default, assumption charts are not shown when you run a simulation. You can change the Show Automatically setting to automatically show assumption charts while a simulation is running or when it stops.
While the Forecast Preferences dialog has a button for fitting distributions to forecasts, distribution fitting is not available in the Assumption Preferences dialog. Instead, there is a Run Preferences button so you can easily change the Store assumption values for sensitivity analaysis setting on the Options tab of the Run Preferences dialog.