You can get the MMI only in certain models, currently the A6 C6, A8 D3 and Q7. If you look at the A3 or A4 you may see references to the MMI system but it isn't the MMI. MMI is an Audi marketing abreviation for Multi Media Interface and is used whereever a system in controlled by the center knob with the 4 buttons around it. The RNS-E has MMI controls build in, the MMI is a multi component system with the MMI controls in the center console.
In the A6/A8/Q7 you can get a MMI Basic and MMI High. If you sit in an A6/A8/Q7 you only see one screen, that's the MMI screen. They also come with the DIS, the display in the instrument cluster. The MMI High comes with the color screen that looks similar to the RNS-E. The MMI Basic comes with a monochrome (red) screen that looks similar to the new BNS 5.0 navigation in the A3/A4.
Today there are the following systems available:
BNS 5.0 (A3/A4/TT MK2), double DIN head unit with CD-ROM navigation, monochrom display
RNS-E (A3/A4/TT MK2/R8), double DIN head unit with DVD-ROM navigation, color display
MMI Basic (A6/Q7?), multiple components (MOST/CAN connected) with monochrome display in dashboard, CD-ROM based navigation may be installed as optional component, controls certain features of the car like air suspension
MMI High (A6/A8/Q7), multiple components (MOST/CAN connected) with colordisplay in dashboard, DVD-ROM based navigation may be installed as optional component, controls certain features of the car like air suspension