What is the difference between internal and external drop ship orders?

Hi All,

I am clear with the case of external dropship, where the supplier directly ships the material to the customer and you(the seller) do only the logical receipt of the goods. 

My understanding of the back-to-back orders,where the material is procured from the supplier first and then shipped to the customer(not directly from supplier to customer). So here we need to the physical receipt of the goods. 

So is internal dropship same as the back-to-back order?

Also what would be the source type on the order line for the internal dropship ? How do i create an internal dropship sales order?