Two-dimensional (2D) materials are intrinsically anisotropic, and an accurate description of their out-of-plane response to an electromagnetic field is more and more important as new materials with diverse properties are proposed. Their electromagnetic properties are often modeled using a single sheet with a surface susceptibility or conductivity or by means of a thin film of finite thickness with an effective bulk permittivity. The discordances between these two approaches lead to two irreconcilable interpretations of the optical characterizations and uncertain predictions of electromagnetic responses. Here, we fully account for the particular anisotropy of 2D materials and reconcile both approaches. We propose a unified description for the electromagnetic properties that applies to 2D heterostructures for all polarizations and at all angles of incidence. In particular, we determine the class of materials for which both models can be used indifferently and when particular care should be taken to select the thickness and the tensorial response of the effective thin film. We illustrate our conclusions on extensively studied experimental quantities such as transmittance and ellipsometric data of graphene and metal dichalcogenides. We discuss similarities and discrepancies reported in the literature when single-sheet or thin-film models are used.