An open model is closed either:
when you select Close from the Application menu or
automatically if you create a new model or open an existing one
Only one model can be open at a time. If the model to be closed has been changed since last save, QPR Modeling Client asks if you want to save changes to that model before closing it. However, in the case you want to have two models open simultaneously, simply start another QPR Modeling Client.
Closing the last model window (diagram, navigator, report, or analysis) closes also the model, and a confirmation is asked before closing the last window.