MRP element data

Dear friends,

I have run MD02 for my finished goods. So it gives me the requirement of raw material list.

I have converted my planned order (for finished goods)in to production order. As soon as i got production order for finished goods, system automatically creates planned order for my raw material. This planned order can be converted as a Purchase request.

My query is related to my raw material planned order.

If i run md04 for my raw material system displays stock/req. list.

It contains MRP Element coloum as: Pldord

MRP element data as:xxxxxxx/STPO.

I can not understand What is "STPO"?

Can anybody describe me what is STPO and its importance?

Thanks

Shri