Although I can't seem to locate a hard definition of "external format" (save for date, time, and timestamp), everything I've seen implies that external formats are human-readable character strings.
(My client doesn't bother with external formats...because its analysts are not required to understand the data
at all! A small group of savvy developers -- such as myself
-- are expected to turn out tools that the rest can use without having to understand icky things like hexadecimal
).
"You have sat too long for any good you have been doing lately ... Depart, I say; and let us have done with you. In the name of God, go!" -- what I say to a junior programmer at least once a day
