What segments are defined as custom in HL7 version 2?

HL7 provides a standard set of segments (like PID for Patient Identity or MSH for Message Header). However, to allow for site-specific data that doesn ' t fit into standard segments, HL7 reserves the letter " Z " for custom segments. Any segment starting with " Z " (e.g., ZPV, ZCD) is considered a local, non-standard extension defined by the specific healthcare organization or vendor.