There are 3 levels in a sales document i.e. header level, item level and schedule line. Line item falls in the Item Level. 1)Line Item relates to material details or the item details such as item number, name of material, quantity etc. 2)The item category of the item which is determined by the system  (by combination of Item Category Group from Material Master, Sales Document Type, Usage and higher level item category)is copied at Item Level of the sales documents. 3)Line Item contains the value and quantity also which are not normally found at Header Level. 4)Examples of Line items are : Material, Order Quantity, Material Group, Plant, Shipping Point, Route, Customer Material, Sales Unit, price/discount/tax/freight per unit, etc. 5)Examples of Header data will make Line Item more clear : Sold-to Party, Pricing date, Document currency, Price group, Customer Group, Incoterms, Payment terms, Purchase Order Number etc.