To Blue Integrator, each message consists of a Message Body and a Message Context. The Message Body is the physical message content. The Message Context is a set of named properties.

Message Context properties can be set automatically on Message Receive or Message Send (configured via the Receive Port Configuration Form and the Send Port Configuration Form), can be set by a Map (configured as output parameters in the Map Editor), or can be set explicitly in VSTA code for a Workflow. Every message has MessageID and Timestamp Message Context properties.  Additionally, different Receive Transport providers populate various Message Context properties as messages are received, such as ReceiveLocationType, FileName and ReceiveFolder.  You can see the Message Body and Message Context Properties using Tracking.

See A Simple Messaging Scenario for an example.