When a segment is optional it appears in square brackets. For example, the message MSH PID PV1 [PD1] indicates that MSH, PID and PV1 segments must appear in the message in this order, and a PD1 segment may appear, as indicated by its enclosed square brackets, after PV1.

	iNTERFACEWARE Products Manual > HL7 Overview > Understanding HL7 Messages > HL7 Segments > Segment Grammar Notation > Optional Segments
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