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
Those APIs are used in Int4 IFTT Boomi Message Selector. In that way a list of potential test case payloads can be searched in Int4 IFTT cockpit and directly the test cases can be created.
For more details you can check out the Boomi specific documentation.