by unit signifies that the column amount is a unitary amount applied in the sales unit entered. Thus it will be multiplied by the quantity.
by line signifies that the column amount is a fixed amount for the line.
by document(only for the sales module) signifies that it is a column that is added up in the invoice footer and for which price list rules by line group exist (for example the discount amount based on the invoice total). The principle of this calculation basis is linked to the use of the grouped price lists. For instance: if, in a grouped price list, rules for the application of a charge have been defined such as for an invoice of which the total amount is greater than €1000, 10% of delivery charges will be applied. The Delivery charges column is defined in the structure code with a calculation basis Par document, and the attached invoice element is Footer delivery charges It the total amount of the invoice is greater than €1000, €100 of delivery charges will be calculated for the footer Footer delivery charges. The price list column Delivery charges is used for the entry of amounts (or percentage according to the setup) that impact the net price of the line and thus will be independently posted on the lines. This setup is used to combine specific/custom grouped price list conditions directly on a footer, whilst conserving the possibility to use the column for another charge or discount on the line.