Each output of the Doug Fleenor 1211-JBOX is electrically isolated from the input, and from every other output, by 2500 volt optical couplers. Each output has its own line driver and associated power supply.
The DMX splitter allows connection of DMX receivers (dimmers, color changers, moving lights, etc.) in a star configuration as opposed to a daisy chain configuration. In a star configuration, each control cable is run to a central point, in this case the splitter. In a daisy chain configuration all the devices are connected on one control cable, the output of one feeding the input of the next. It is not good practice to simply split a DMX control cable using a "wye" cable as this can cause signal corruption due to reflections. Typical uses of the splitter include splitting the control signal between stage right and stage left dimmers, running an isolated split to a string of color changers, splitting and isolating a feed to a wall outlet for temporary or rented dimmers, isolating a stack of dimmer packs from each other, etc