Debit and credit notes

Debit and credit notes : A debit note is given by a purchaser to a seller when the purchaser returns the items back to the seller and a credit note is then given to the purchaser by the seller.
A transaction that reduces the amt recievable from the customer is a credit memo and the one which reduces the amt payable to a vendor is a debit memo.
It is basaically a sales doc which is used in complaints processing to request a credit memo for a customer. A credit memo can be used so that it can be assigned to the relevant open invoices and carry out clearing with them.
Dunning program : WHy are we taking about one here. If a credit memo is assigned to an invoice and if it completely not able to offset the credit memo against the invoice then it is submitted as a debit memo to a vendor who is to reimburse the amt.after this a multilvele dunning program can be used.
A dunning program lets you dun customers who are making late payments
F150 is the tcode for dunning program and it basically outputs to give dunning forms.
To do the same process in CRM that is to create debi/credit memo in ECC we need to create complaints.But to create a complaint basing on an Invoic reference u have to use a badi CRM_COPY_BADI_EXTERN (check spro) this badi helps you give invoice references but only the ones in crm for the ones in ECC got to write some code with a remote function module which fetches data from ECC.