The CC is a byproduct of the FC oxidizing the (presumptively) ammonia in your pool. It's always an indication that there's something in the pool that's being oxidized by the chlorine.

The goal of shocking is to get rid of stuff in your water that doesn't belong there. When you can hold (less than 1 ppm loss) FC overnight (no loss to sunlight) and measure nearly no (0.5ppm or less) CC for two nights in a row, you'll have have finished shocking (gotten rid of unwanted pool stuff).