Using first-principles calculations with a hybrid functional, we examined several fundamental issues for heterojunction structures formed by the same material but in different polytypes including the precise location of the boundary and the polarization. Particularly, we demonstrate that the inclusion of 8H in 4H SiC generates a single quantum well (QW) structure, rather than a double well consisting of two 3C regions separated by a single hexagonal layer barrier. The level of the QW states for 8H and 3C inclusions are calculated to be 0.42 and 0.68 eV, respectively, below the conduction band minimum of 4H SiC, in good agreement with the experiments.