When a prepayment invoice is created from a sales order, the 'compress prepayment' field manages whether 1 (GL account) invoice line should be created for the entire sales order (compress = YES), or 1 line per sales order line (compress = NO).

If compress = NO and 1 line per sales order line will be created, the (item) description from the sales order lines will be copied to the invoice lines. However, the quantity in the invoice line will always be 1, and the amount will be the total calculated prepayment amount .

Customers frequently ask to have the quantity from the sales line copied to the invoice line. Either as quantity, or in a new field.
Also frequently asked is to have the item no. from the sales line copied to the invoice line, in a new field.