pub.sap.idocToString Information

We have more than one EDI_DC in outbound IDOC String. In Mapping, I am using decodeString, decodeSDATA, idocToString. First two are working fine. But idocString only works for first EDI_DC and discard rest of the EDI_DC.

Any Suggestions?

Also, Where can I find the details of pub.sap.idocToString flow service ?

Thanks
Chetan