The ZLR segment must follow each OBR (Observation Request) segment, and there can only be one ZLR per OBR.
HL7 version evolution
The PCG Platform Interface / HL7 Engine
PIX/PDQ Processing Flow
JWG-CDM model development strategy
The HL7 RIM (0.92), 1999
HL7 Binding Component as a Consumer
Healthcare Information System