Forecast consumption question

We are using MRP to create planned orders.  Our forecast set has a bucket type of periods.   If we use 0 for both backward and forward consumption days, does that mean that the forecast will only be consumed if the forecast date is in the same period as the scheduled ship date of a sales order?  If this is true, where do I find the definition of periods - ie how can I tell if the dates are in the same period?