Editing EDIFACT (section)

== Structure ==
EDIFACT has a hierarchical structure where the top level is referred to as an ''interchange'', and lower levels contain multiple ''messages'' which consist of ''segments'', which in turn consist of ''composites''. The final iteration is an ''element'' which is derived from the United Nations Trade Data Element Directory (UNTDED); these are normalized throughout the EDIFACT standard.

A group or segment can be mandatory (M) or conditional (C) and can be specified to repeat. 
For example :
 - C99 indicates between 0 and 99 repetitions of a segment or group
 - M99 signifies between 1 and 99 repetitions of a segment or group

A group, like a message, is a sequence of segments or groups. The first segment or group beneath a group must be mandatory, and the group should be made conditional if the logic of the situation demands it.

  |_Service String Advice              UNA  Optional
  |____Interchange Header              UNB  Mandatory
  :    |___Functional Group Header     UNG  Conditional
  :    :   |___Message Header          UNH  Mandatory
  :    :   :   |__ User Data Segments          As required
  :    :   |__ Message Trailer         UNT  Mandatory
  :    |__ Functional Group Trailer    UNE  Conditional
  |___ Interchange Trailer             UNZ  Mandatory