As per Wikipedia, the UNICAST-RESPONSE field is used to minimize unnecessary broadcasts on the network: if the bit is set, responders SHOULD send a directed-unicast response directly to the inquiring node rather than broadcasting the response to the entire network.
What does it mean when it's set in response? Wireshark shows it as Cache flush, what does that mean? I couldn't find any more documentation.
TIA.
Read the next section on the wikipedia page you link to:
The CACHE-FLUSH bit is used to instruct neighbor nodes that the record should overwrite, rather than be appended onto, any existing cached entries for this RRNAME and RRTYPE.
The bits in the query and reply does not carry the same meaning, even though they may appear at the same position. It's two different message formats, and you can't use either one to infer anything about the other.
The function is further explained in RFC 6762
