The segment at the end of an EDI transaction that specifies where the EDI message ends.

Merriam-Webster Online Dictionary
trailer (noun)
1.
one that - trails
2.
a plant - trailing
3.
a nonautomotive vehicle designed to be hauled by road as
a) a vehicle for transporting something - a boat trailer , especially - semitrailer
b) a vehicle designed to serve wherever parked as a temporary dwelling or place of business
c) - mobile home
4.
a) - preview
b) a short blank strip of film attached to the end of a reel
trailer (verb)
intransitive verb
1.
to live or travel in or with a trailer
2.
transitive verb
to be transportable by trailer - a light boat that trailers easily to transport (as a boat) by means of a trailer