Int4 IFTT utilize the following Boomi standard APIs to fetch the message payloads for test case creation as well as during test case execution.
ExecutionRecord - get a list of process executions
ExecutionConnector - for a given executionId get a list of connector shapes used in the process
GenericConnectorRecord - for a given executionConnectorId get a list of documents tracked on that shape
ConnectorDocument - for a given genericConnectorRecordId download the actual document payload
For more details you can check out the Boomi specific documentation.