Hello Sahar,
Let me try to give you an overview.
Price discounts - not actually discounts. You can specify a price but not a discount (neither a percentage discount, nor an amount discount). Use Price agreemens to enter any prices you may have on price lists for individual customers, groups of customers or all customers. You can set up different prices depending on quantity ordered as well. DAX puts the price from this type of agreement into the Unit price field on the Sales order. This is the starting point for calculating the final price for the customer.
Line discounts - here you specify discounts as percentage or as an amount. The discount is applied to the Unit price, i.e. the price from the Price agreement above. You can also set up different discounts depending on quantity ordered. With this type of trade agreement DAX only takes into account the quantity ordered on a single sales order line. Example - I sell Coke, Sprite and Fanta and I want to give my customer a 10% discount if they buy 3 bottles of one product(I give athe discount when they buy 3 bottles of Sprite or 3 bottles of Fanta but I won't give the discount when they buy 2 bottles of Sprite and 1 Fanta).
Multi line discount - similar to Line discounts. The difference is DAX takes into account all eligible lines from the ame sales order. So you can set up discounts for quantity based on the total ordered quantity of several items - you cannot do that with the Line discounts. Example - I sell Coke, Sprite and Fanta and I want to give my customer a 10% discount if they buy 3 bottles (I don't care if they buy 2 bottles of Sprite and 1 Fanta or 3 bottles of Sprite only, I still give them the discount).
You can combine Line and Multiline discounts - look at the Discount setting in Account receivable -> Setup -. Parameters -> Prices tab.
Total discount - use it to specify one discount for the whole sales order. It is applied to the total amount of the sales order. Use it when you want to give a discount depending on the amount of the sales order and not on the quantity of items ordered. Line and Multiline discount take into account the quantity, Total discounts take into account the amount (the criteria for applying a discount). Another important difference is that with Line and Multiline discounts you can callculate the margin for each sales order line. The total discount can be used to calculate the margin on the sales order header only, it cannnot be broken down into the lines.
Pay attention to currency issues as well. Trade agreements are always tied to a certain currency. So if you set up a trade agreement in USD for a customer usually placing orders in USD you will get a problem if the customer tries to place a sales order in EUR - they will not get the price/discount from the trade agreement.
Good luck,
George