Eyco,
The Magic 260 can control up to 24 individual fixtures by assigning them to individual buttons. Using the multi fixture option allows you to control more than just one fixture per button. Each fixture button has a maximum number 16 channels, so this option would be used with fixtures that have 8 channels DMX or less. For example, when using 3 channel RGB fixtures, you can individually control a total of 5, from one fixture button. As opposed to using up 5 fixture buttons. It also speeds up programming by having quick access to more fixtures.