MDI is the abbreviation for Multiple Document Interface. It can be specified in the IBExpert menu item Options / Environment Options.
In IBExpert this is the recommended interface, as all windows are contained within one main Window, similar to MS applications. There is one document per window. For all additional objects or documents, the Windows operating system opens an additional window.
The status bar can be seen at the bottom of the screen.
When changing the interface from SDI to MDI and vice versa, IBExpert needs to be restarted for the alterations to take effect.