Diffraction can be understood as a special reflection. What's the difference between diffraction and true reflection? Why?

The diffraction here refers to Bragg diffraction. When light waves pass through the lattice, because there are many crystal planes, it can be regarded as the coherent superposition of many special reflections produced by many crystal planes. Only when the optical path difference between these light waves is equal to an integer multiple of the wavelength will superposition occur, and then we can observe the diffracted light waves.