Surprisingly this is something that is not clear for most designers, although the concepts have been around since late 80’s/early 90’s and often bandied about in professional tones of mastery–Only to be humbled by realizing the fallacies! Also this is not something exactly taught in design school (ahem). However, here’s the basic breakdown of what these concepts mean and why they are confusing…
MDI: Multiple Document Interface, meaning you have many document windows, with the menus, tool bars, palettes all separated out into their own windowed objects, all within the application container, or environment
SDI: Single Document Interface, meaning the document is consolidated into just one window that has the menu, tools, tabs, etc. all compacted into that windowed object (ie, the application container = the window object, with all functionality self-contained)
So what does all that mean, regarding Mac and WinXP?
1. Pseudo-MDI: On WinXP, each application’s document has a window but there is a general application level menu bar, tool bar, etc. On WinXP, closing the last window exits the app. However, on the Macintosh, because the application menu bar is shared with the Finder menu bar, there’s some confusion. You can have multiple document windows open but only one set of menu, toolbar, palettes that contextual update based upon which document is in focus.
2. True SDI: IE is pretty much the exemplar, with each IE window being self-contained, with its own menu, toolbar, history panel, etc.
3. True MDI: X Windows and NEXT, in which even the menu or document navigator is its own window floating with the document windows.