Just because I'm a really nice person
The Midline/ Highline refers to the computer insert between the rev counter and speedo and it's abilities / functions.
Standard models are uisually fitted with the Midline. This can be identified by the fact that half the insert mention above (the lower half) is an LED display that shows you Outside temperature, MPG, miles to empty etc. The other half of the display are fixed warning lights (i.e. low fuel, etc). There is a black plastic bar that seperates the 2 parts of the display (see image below for midline display).
The highline display insert is all one piece and is totally LED. It performs all the functions of the midline display but is capable of much more. It has a series of menus that be toggled through and can display fuel computer stats, Audio menu (track playing, FM station name etc), Navigation menu (displays the turn by turn warnings on this display as well as on the Navi unit. Also, it allows you to control certain aspects of the cars behavoir, that's normally only able to be set via VCDS (VAG-COM). Items such as Headlight delay, one touch opening, auto locking over 5mph (and unlocking), winter tyres, swtiching units (°C to °F, 12h or 24h clock) etc.
As you can see the highline display is much more versatile than the midline and only costed about £50 to add as an option at order time. Problem is it wasn't an option many dealers new much about so many cars were specced without them.
For info, the highline display looks like below :
Andrew