Hi,
Table Conversion and flat file conversion are pretty much the same : you export data from your own ERP to some .TXT
or .CSV file, those files are read by some custom JDE
Table Conversion program (written by you) and data is
written directly into JDE tables.
That approach can be quite risky because you directly
insert data without running any prior validation.
Interface tables (or also known as Z1) are temporary
tables which you have to fill up (typically by SQL
INSERT queries or massive database importing from
text files or by table conversions too).
Once those Z1 tables are filled up, you have to run
JDE batch processes that will validate their contents
(you'll run them several times until the validation is OK)
and finally will be imported into master JDE tables.
I personally prefer the Z1 approach, at least it provides
some validation on what you're inserting.
Most of this activity is disk-intensive (reading and
writing records), so I doubt you may have any performance
gain by writing the import process with C APIs instead of
going through easier JDE toolkit tools.