When you first create a STEM model, using the New command from the File menu, the model is given a temporary name, such as Model 1, and initially just contains global data with default values, e.g., Model Run Length = 11. As you start to create model elements and to populate the input data for those elements, it quickly becomes imperative to save the model to disk, as the model is only stored in computer memory up to this point.
Note: When you are working in the Editor, it is wise to save your model(s) frequently, to minimise the risk of losing significant changes in the event of a power outage or similar mishap.