A map tells translation software where to find each piece of data and where to put it. The purchase order number in the BEG segment of an 850 becomes the order number in your ERP; the item and quantity in each PO1 segment become order lines. Maps also convert codes, units of measure and date formats, apply defaults and handle repeating groups such as multiple ship-to locations.
EDI translation is the process that runs the map: it reads an inbound ANSI X12 or EDIFACT file, checks its syntax and produces the target format, or does the reverse for outbound documents. Mapping is the design work; translation is the execution. A mapping specification documents the rules, usually based on each trading partner’s implementation guide.
Mapping is where much of the cost of EDI sits. Each trading partner’s version of a transaction set can differ, so companies often maintain many maps for the same document. When a trading partner changes its requirements or you change ERPs, maps must be updated and retested. A map can also produce a perfectly valid file with the wrong data, which is why testing and certification matter.