No difference. When a radio maufacturer designs a HU with 18 Fm presets, s/he doesn't want to have eighteen buttons on the face, one for each preset. So they usually put them in sets of six. Most people never listen to more than six channels in any one area anyway, so most people use the different blocks (FM1, FM2, etc) for different parts of the country. E.g., I sue FM1 when I am at home (in Chicago), and FM2 when I visit my parents (in Indiana).