I wasn't able to find anywhere in the v2 specs that give a clear description of how the concept of a Set ID (NTE-1, OBX-1, etc.) is supposed to work.  The primary question this leaves open is whether Set IDs should restart for each segment group (for example, ORU^R01 allows NTEs after PID, OBR, and OBX - should the NTE-1 values always increment through the whole message, or restart for each group?).  The proper behavior should probably be called out somewhere in the Control section and referenced by each field that is a Set ID.

Additionally, most references for Set ID I looked at were different; and some remarkably unhelpful:

• NTE-1:  "Definition: This field MAY be used where multiple NTE segments are included in a message. Their numbering must be described in the application message definition."  (I didn't see that last part actually happening)
• OBX-1:  "Definition: This field contains the sequence number. For compatibility with ASTM."
• PID-1:  "Definition: This field contains the number that identifies this transaction. For the first occurrence of the segment, the sequence number shall be one, for the second occurrence, the sequence number shall be two, etc."
• OBR-1:  "Definition: For the first order transmitted, the sequence number shall be 1; for the second order, it shall be 2; and so on."
• SPM-1:  "Definition: This field contains the sequence number. This field is used to identify SPM segment instances in message structures where the SPM segment repeats."
• IN1-1:  "Definition: IN1-1 - set ID - IN1 contains the number that identifies this transaction. For the first occurrence the sequence number shall be 1, for the second occurrence it shall be 2, etc. The Set ID in the IN1 segment is used to aggregate the grouping of insurance segments. For example, a patient with two insurance plans would have two groupings of insurance segments. IN1, IN2, and IN3 segments for Insurance Plan A with set ID 1, followed by IN1, IN2, and IN3 segments for Insurance Plan B, with set ID 2. There is no set ID in the IN2 segment because it is contained in the IN1, IN2, IN3 grouping, and is therefore not needed. The set ID in the IN3 segment is provided because there can be multiple repetitions of the IN3 segment if there are multiple certifications for the same insurance plan, e.g., IN1 (Set ID 1), IN2, IN3 (Set ID 1), IN3 (Set ID 2), IN3 (Set ID 3)"