Conventionally, the Poynting vector as EM power flow has been thought to be a well-established basic concept in textbooks. In view of the existence of a certain mathematical ambiguity for this conventional basic concept, Mansuripur and Zakharian suggested it to be a "postulate", while Stratton suggested it to be a "hypothesis", "until a clash with new experimental evidence shall call for its revision". However, this basic concept is challenged in a recent study, which claims "Poynting vector may not denote the real EM power flow in an anisotropic medium", and "this conclusion is clearly supported by Fermat's principle and special theory of relativity".