During a session, the Integration Service creates a reject file for each target instance in the mapping. If the writer or the target rejects data, the Integration Service writes the rejected row into the reject file. The reject file and session log contain information that helps you determine the cause of the reject.
Each time you run a session, the Integration Service appends rejected data to the reject file. Depending on the source of the problem, you can correct the mapping and target database to prevent rejects in subsequent sessions.