Consider the following sentences from the research paper titled PatternNet: Visual Pattern Mining with Deep Neural Network by Hongzhi Li et al.
The value of each pixel in a feature map is the response of a filter with respect to a sub-region of an input image. A high value for a pixel means that the filter is activated by the content of the sub-region of the input image.
The sentences mention the sub-regions of an image. Is there any formal definition for a sub-region of an image?