ONAP Logging Guidelines (https://wiki.onap.org/download/attachments/1015849/ONAP%20application%20logging%20guidelines.pdf?api=v2) talks about having a Request ID as a "a universally unique value that identifies a single transaction request within the ONAP platform" and using it for "tracing of the flow of processing that tells what ONAP components and sub components are involved 
in its processing".

As per the guidelines, the RequestId should be in header section of REST API, but for Request REST API of APPC, which is called from POLICY, it is part of payload section.