PT_USER_EXIT_DART Code

Dear All,

I am working now on SAFT-PT Report i did validate the form for SD documents

but when i include the FI invoices it shows error type in the validator..

I raised a msg to SAP and they said you have to map the invoice type using the user exit

PT_USER_EXIT_DART.

Any ideas about the code that i need to write and for what transaction type ?