A single EDI transmission constitutes an ISA/IEA envelope, which isolates one group of transmitted data from another. Each envelope contains EDI documents to (outbound) or from (inbound) a single EDI trading partner.   These envelopes contain one or more GS/GE envelopes, which contain one or more documents of the same type.  The use of separate GS/GE envelopes allows for several different document types to be communicated within the same ISA/IEA envelope. Within the GS/GE envelope is the ST/SE envelope that isolates one document from another.  A Transaction Set is wrapped in the Transaction Set Header/Trailer (ST/SE) envelope.  Data Segments (similar to a record in a data file) are used to make up Transaction Sets.   Data Segments are roughly equivalent to a single line on a document.  Data Elements (similar to a field in a record) contain the basic units of information and are used in various combinations to make up Data Segments.

The Interchange Header (ISA), commonly known as the Communications envelope, starts an interchange of one or more functional groups.  It contains: the syntax identifier, Sender and Receiver EDI ID's, unique control number, acknowledgment requested, test indicator, date and time sent, recipient reference or password, and version of Standard being used.  

The Functional Group Header (GS) indicates the beginning of a group of transaction sets and provides a second level of Sender and Receiver IDs.  It usually includes an identifier for the type of transaction set, application sender identifier, application recipient identifier, date and time sent, unique functional group reference, agency responsible for specification of structure (i.e. X12, VICS), message type version, and password.

The Detail area contains the details of the business transaction.   In a PO document, the ST includes elements such as: Purchase Order Number, Date and PO Type, Item Numbers for Products Ordered and Product Descriptions, Prices, Method of Shipment and Delivery Dates, Terms of the Order, and, for some orders, a Distribution List by Item by Location.

The Trailer Envelope Segments contain control information such as total ST Segments and total GS Segments and unique control numbers that must be identical to the values in the ISA and the GS.

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