Messages are the lifeblood of every BizTalk solution. Any data, regardless of type, is represented as a message within BizTalk. Typically, messages are represented as XML documents, although you can also handle messages represented in binary format. Doing so, however, will limit the features you can use. For example, you cannot use the BizTalk Mapper or Rules Engine on binary-formatted messages.
A BizTalk message has at least one underlying part, which is typically the body of the message. If you want to hold multiple pieces of data within one message, you can create a multipart message.
BizTalk also offers the ability to track message bodies, which can then be viewed using the Health and Activity Tracking (HAT) tool. HAT is not enabled by default, as it introduces significant overhead and should be used only if absolutely necessary.