Stock Type is a mandatory field. All normal stock items are Stock-N-Sale items.
All items that are of intangible nature like Freight, Service Charges, Custom Duty, etc and therefore, do not have stock, are Non-Stock items.
Indent items are like Stock-N-Sale items but you usually order them only when you receive an order for them. These are items that you do not want to stock physically.
Manufactured items are those that you or assemble internally and normally never buy from the market.