High is for cars equipped with the MK4 DVD Navigation computer based on the i-bus platform. So that will be the E46 3-series, the X3, Z4, old 5 series, old X5 series etc. These cars usually have the nav computer/drive in the boot somewhere.

Professional is for i-drive equipped cars, such as the 1 series, new 3 and 5 series, the new X5 etc. They usually have the Nav drive in the dash

Business is the monochrome 'Radio based' navigation which will have a Mono equipped Mk4 navigation computer but no colour screen. They don't have a map and just show arrows/distances on an orange display on the radio but do have the usual voice prompts.

For an X3 with a DVD drive, you will need HIGH. If your Nav Computer hasn't been upgraded since 2004, for the newer DVD's to work (after 2006 probably) you will need the Nav upgraded. Go to the SET menu and check to see what the version numbers at the top of the screen. Mosey over to the Navigation forum for info and locations to get the ISO image for the update CD.

HTH.