We consider the infrared (IR) aspects of S-matrix in QED. We will review IR divergences and their relation to the asymptotic symmetry in QED, and introduce the dressed state formalism to obtain IR-safe S-matrix elements. We show a generic condition (dress code) for dressed states to obtain IR-safe S-matrix elements, and explain that the dress code can be interpreted as the memory effect and represents the superselection rule of asymptotic symmetry.