The arrangement of data in an EDI transaction set, as defined by the Standard being used.

Merriam-Webster Online Dictionary
syntax (noun)
a) the way in which linguistic elements (as words) are put together to form constituents (as phrases or clauses)
b) the part of grammar dealing with this
a connected or orderly system harmonious arrangement of parts or elements - the syntax of classical architecture
especially as dealing with the formal properties of languages or - syntactics calculi