John,
Answer for your Qn is "NO"
SPP will not consume forecast (PIR) when a SO created on the same time period bucket.
SPP will not consider future (delivery date in future) SO unless you configure and ask SPP to consider it.
SPP just forecasts to find out the optimal future demand based on your past sales histories and to find the forecast error (std deviation) so that it will be an input for further planning in SPP.
SPP plans proactively and will ask you to keep enough stock in advance so that you can serve the moment you get SO. its always procure to stock --> sell from Stock. it will not consume SO against forecast as Make to stock type of planning (Forecast consumption by SO will not work for procure to stock).
In SPP
Today -----------------Forecast Qty - 100
Tomorrow------------ Yesterday forecast (100) will be vanished and its not considered as requirement anymore..
Just my thoughts
Regards
Vinoth