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This field is useful when there are a large number of Send HTTP Response activities in a job. In such cases, the Close Connection field is checked in the last activity to indicate the response is complete. However, checking the Flush Response field overrides this behavior and causes the response to be flushed after each Send HTTP Response activity.
Basically all responses are kept in a buffer. The default buffer size on the server side (BW) is 8196 bytes and this is currently not configurable. When "Flush Response" field is checked, BW flushes the stream after every chunked response that is sent back to the client.