The design applies a Sobel operator to extract the image gradient intensity. Canny edge detection often uses a Sobel operator as its kernel weights have a degree of noise suppression.
The Sobel output is a 2D vector matrix in which its magnitude represents edge intensity strength and its direction indicates the orientation of the edge direction.
For more information: Rafael C. Gonzalez, Richard E. Woods Digital Image Processing Second Edition, 2nd ed., Prentice Hall pp.577-578 andl pp.579
The edge magnitude may exceed the maximum 8 bit pixel value of 255 so the design saturates the edge output pixel to 255 rather than normalizing, which does not give a very bright image output. Thus the Sobel output flushed to the monitor screen is saturated. The subsequent Canny blocks use the full Sobel output bit range for maximum precision.