In a diet-allowed nutshell: the SC diet is intended to help severe GI problems. The basic no-no's: no grains of any kind (including grains-that-aren't-really-grains like Kasha), no starchy veggies like corn or sweet potatoes, no sugars except honey, and no soy products. There are almost no mass-produced products allowed either, so pretty much everything needs to be made from scratch. Allowed are most fruits, vegetables, animal sources of protein, and flours made from nuts.
Our sweet Chana (age 8 1/2) was recently diagnosed with Crohn's disease and spent a week in the hospital. The choices were: 1) a LOT of medications with a LOT of difficult/nasty/long-term side effects or 2) the SC diet with LESS medications (and, hopefully, in the future "no medications").
So here we are with her in her 9th week of the diet (not that we're counting every day or anything :)).