Message output systems

Hi Gurus,

I am a Functional Consultant and do not know much about the ABAP. I am making a Functional specification for the change Message out put which is for Remittance advise for the Vendors. I read few of the Threads an I am sure now I can get some help here.

The scenarios is that the ERS (auto MIRO or invoice generation) is done with the help of self billing (automated). This has 7 different steps in which it calculated and the creats and invoice document. I have tried to view the program and at the end it has RSNAST00 program.

Now I am a bit confused here as there is another custom program "ZPMWO_SBHS_MESSAGE_NAST" i went through the code for this and I think this program is to fetch the data that is to be populated in the Smart forms as at some point in the program the smart forms are called and populated with Data.

As this whole thing is confusing to me My basic question is whats the role of RSNAST00 when data is fetched by the Custom program. Is it thats the custom program is getting the data and populating the Smart form and then creating the message and RSNAST00 is then Checking the NAST table for the unprocessed messages and picking up unprocessed messages and send to the respective vendors.

Can anyone please explain me the purpose of RSNAST00 and how it can be used and if the above scenario makes sence or not?

thanks in advance

Anoop Sharma