Sales Forecast Coefficient and SIOP

Hi Experts,

What is the use of Sales Forecast Coefficient ZP26 and why do we maintain sales forecast in ZP27. For SIOP we create product group in MC84.Is it the same as creating Product hierarchy? If different in what way it is.

Please also help me in getting familiarized with the SIOP process step by step, with explanations of the why of the individual steps. I will come back in case of any further doubts in completing the SIOP scenario. Even Google has very little inputs on SIOP.

Thanks

Sakthi