It is easy to forecast how many widgets you can ship when
you are creating them in the factory behind your office. It is another story
when those widgets are being created in another factory that you do not own.
You are now a customer of your distributor, but as a market economy would
dictate, your customer's do not care where the product comes from. They just
want to know if you have it in stock, and if you do not, they would like to
know how soon you will have it. Any company that ships a product that they do
not produce themselves runs into this problem consistently, and often enough
you will need to connect you systems to your distributors to get a virtual
inventory to keep your customer happy.