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• The objects returned by CascadeClassifier::detectMultiScale are cv::Rect (or vector of 4 int in Python)
• I suppose that it is an old doc (by the way, the link is for your question, not the docs), rejectLevels are for output as the docs say. An they will contain some numbers, if you set the flag outputRejectLevels; but I do not remember exactly what they mean, there is no doc on it, but you can test it.

I can tell you that I have used once rejectLevels or the levelWeights for having the best face-detection, but it was not as good as the groupRectangles function...