When you have used the scenario tree (see Quick Analysis with Scenario Tree) to build a scenario via parameter changes or to load a pre-existing set of parameter changes (an .sce file) and then run the scenario to generate a run or results file, the Working File will hold the contents of that model run. It will therefore be different from the Base Case, to which it is automatically set every time you start IFs again. Should you wish to reset the Working File to the Base Case in a session of work with IFs, simply use the Reload the Base sub-option under the Scenario Analysis option of the Main Menu.