Hi Sandeep,
Demand - whats the total quantity ordered for that particular month. Say 3 sales order received requesting 30 qty in each for the month of APR 2014. Then your history (demand) roughly should be 90 when you see it at APR 2015.
Item - in above case, no of times we order is 3 for APR 2014. So your ITEM history is 3 on APR 2015.
Demand/Item is 90/3 = 30, same would be seen as Demand/ITEM history for APR 2015
To your question, 1000 is the total order qty for the month AND 2 is the no of SO.
Yes Poison Distribution uses the Demand/ITEM and then Demand in peaces as its required for complex calculation which suits for sproadic demand (Volatile).
Hope this answers your Question.
Just my thoughts.
Regards
Vinoth