It stands for either Constant Quality or Constant Quantizer. It's a way to keep the quality (or quantization) of the encoded frame at a constant, without regard to bitrate, whereas CBR and VBR will reduce the quality in an attempt to prioritize the bitrate. CQ modes are found in outside encoders like HCEncoder for MPEG2 and x264 encoders for H.264. The Rate Factor you see in the screenshot defines the Constant Quality. 23 is actually pretty damn good. 0 would be lossless, with the quality going down as the number goes up. 53 is the highest setting and the lowest quality.
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