HL7 standard supports primarily two modes of operations: Batch and Interactive. Data exchanged between systems can roughly be classified into two groups: individual and bulk to suit the two modes of operations.
By definition, bulk message is a collection of individual messages. It consists of header, sequentially listed individual message, and trailer. The embedded individual message retains its structure, just as it were in the interactive operation mode. There is no structural difference between individual messages operating in two different modes of operation. But because the nature of batch operation does not allow dialog messages, certain categories of individual message are only suited for the interactive operation mode.
Structurally all individual message contains a header. Some contains body and others don't. An Individual message is, according to the standard, an "atomic unit of data transferred between systems". However the very same standard also allows the notion of logical message, whose data is physically broken down to more than one individual messages and correlated together using a logical message id in message headers. The breakup of a message into individual messages is driven primarily by message length negotiated between parties engaging in message exchanges.
For more information about the HL7 message structure and HL7 message components see