RE: AR debit memo/cash sale & AP credit adjustment (dynamic SL)
You might use an AR debit memo to add to what a customer owes. While a debit memo and an invoice are basically the same, using a debit memo for an adjustment instead of an invoice helps differentiate adjustments from actual invoices. You might also use a debit memo if you enter a credit memo incorrectly or the credit memo needs to be reversed. For example, you may have used a credit memo to write off a bad debit and then you received a payment on some or all of what you wrote off.
An AR cash sale basically creates the equivalent of an invoice and a payment application. A cash sale is typically used when the payment is received at the same time the goods are sold or for transactions such as vending machine income.
An AP credit adjustment might be used similar to what you might use an AR debit adjustment. It is basically the same as a voucher but, like indicated above on an AR debit memo, using an AP credit adjustment helps differentiate that it is an adjustment as opposed to a new voucher.