Bad files contains column indicator and row indicator.
Row indicator: It generally happens when working with update strategy transformation. The writer/target rejects the rows going to the target

Columnindicator:
D -valid
o - overflow
n - null
t - truncate
When the data is with nulls, or overflow it will be rejected to write the data to the target
The reject data is stored on reject files. You can check the data and reload the data in to the target using reject reload utility.