Description for EDI Envelope Structure

The envelope structure hierarchy is as follows:

UNA: Service String Advice (Optional)
UNB: Interchange Header
- UNG: Functional Group Header (Optional)
  - UNH: Message Header
    - Data Segments
  - UNT: Message Trailer
  - … Additional UNH - UNT pairs
- UNE: Functional Group Trailer (Optional)
- UNG: Functional Group Header (Optional)
- … One or more UNH - UNT pairs
- UNE: Functional Group Trailer (Optional)
UNZ: Interchange Trailer