This field displays, in the currency of the sales order, the prepayment amount that will be deducted from the next ordinary invoice for this line.
When you create an invoice for the order, the program suggests an amount proportional to the amount of the order that is being invoiced. For example, if prepayment invoices have been issued for the order for LCY 10, and you invoice half of the order, the default contents of the field will be LCY 5.
You can change the amount to deduct before you post the sales invoice.