A phantom BOM is a bill-of-material coding and structuring technique used primarily for transient non stocked) subassemblies. For the transient item, lead-time is set to zero and the order quantity to lot-for-lot. A phantom BOM represents an item that is physically built, but rarely stocked, before being used in the next step or level of manufacturing. This permits MRP logic to drive requirements straight through the phantom item to its components, but the MRP system usually retains its ability to net against any occasional inventories of the item. This technique also facilitates the use of common bills of materials for engineering and manufacturing. The rule of the phantom, as a general rule, is that you do not stock it, you make it when you need it.