SDI is a Single Document Interface. As opposed to MDI which is a Multiple Document Interface. the difference is solely in the number of documents that can be opened at one time in each instance of the application. With SDI only one document can be open whereas with MDI many documents can be open.

MDI stands for multiple document interface, more information and an explanotion can be found on wikipedia. Multi document interface is the opposite of singe document interface.

The multiple-document interface (MDI) allows you to create an application that maintains multiple forms within a single container form. Applications such as Microsoft Excel and Microsoft Word for Windows have multiple-document interfaces.

Multiple Document Interface.....................by shiv

This Interface Control Document (ICD) defines the requirements related to the interface between the Space Segment (SS) of the Global Positioning System