What does the variance mean in the PriorBox of SSD？

[GENG YANG]

My question is as the title, and I find it strange that in the source code "prior_box_layer.cpp", the top_data containing the coordinates of default boxes is replaced by the values of variance. What does this part of code mean?

layer {
  name: "conv9_2_mbox_priorbox"
  type: "PriorBox"
  bottom: "conv9_2"
  bottom: "data"
  top: "conv9_2_mbox_priorbox"
  prior_box_param {
    min_size: 264.0
    max_size: 315.0
    aspect_ratio: 2
    flip: true
    clip: false
    variance: 0.1
    variance: 0.1
    variance: 0.2
    variance: 0.2
  }
}

// set the variance.

  top_data += top[0]->offset(0, 1);

  if (variance_.size() == 1) {

    caffe_set<Dtype>(dim, Dtype(variance_[0]), top_data);

  } else {

    int count = 0;

    for (int h = 0; h < layer_height; ++h) {

      for (int w = 0; w < layer_width; ++w) {

        for (int i = 0; i < num_priors_; ++i) {

          for (int j = 0; j < 4; ++j) {

            top_data[count] = variance_[j];

            ++count;

          }

        }

      }

    }

  }

}