In the product-site record there is a demand horizon.  

From my understanding, anything before that time MRP considers Firm SOs and anything after is Firm SOs and Forecast Demand.  For me this makes sense for the products that are manufactured

What is it's behavior if the product is purchased?  Is it that when the purchased products are influenced by the many different finished products' forecasts and SOs when MRP is calculating whether or not to create a PO ?

Are POs and WOs part of the demand horizon also ?