A field is a string of characters. HL7 does not determine how systems store data within an application. When fields are transmitted, they are sent as character strings. Except where noted, HL7 data fields may take on the null value. Sending the null value, which is transmitted as two double quotation marks (""), is different from omitting an optional data field. The difference appears when message content is used to update a record in a database rather than to create a new one. If no value is sent, the old value is unchanged. If the null value is sent, the old value is changed to null.

A composite may consist of either a primitive data type (ST,ID, and so on) or a group of subcomposites.

Subcomposites are parts of a component and consist of primitive data types (ST, ID, and so on).