Hi. I found this short explanation about the difference between I3C and DIM.
Absorbable DIM formulations overcome the need for active enzymes within the vegetable and chemical reactions in your stomach to produce DIM. For similar reasons, absorbable DIM provides many advantages over indole-3carbinol (I3C), another cruciferous phytochemical available as a supplement. I3C is an unstable precursor that requires activation in the stomach to be converted into DIM. This means I3C must be taken at a much higher dose and can undergo unpredictable and undesirable chemical reactions in your stomach and colon. DIM, in a delivery system to assure absorption, is by far preferable to the supplemental use of I3C.