From APM documentation > Configure Correlated Tracing section:
"Cross-process transaction tracing requires the agent to insert a correlation identifier that can be passed from one process to another. The agent can insert this correlation identifier into either a SOAP or HTTP header.
Because most services use SOAP messaging, the correlation identifiers are inserted and read from SOAP headers by default. In certain situations, however, you may prefer to pass the correlation identifier using HTTP headers. For example, in rare cases, adding the correlation identifier to the SOAP header can cause the message to be rejected because the correlation ID changes the SOAP payload.
Depending on how your applications process SOAP messages and how you have implemented security, you can choose whether to pass the correlation ID in the HTTP header or the SOAP header .. "