DERs introduce a substantial number of variables and constraints, making their direct integration into high-level dispatch processes highly complex. To address this issue, VPPs have emerged as an effective solution by treating diverse DERs as a single entity and employing aggregated flexibility envelopes to reduce the dimensionality of variables and constraints, thereby streamlining upper-level optimization. This chapter presents a steady-state model to represent the flexibility regions of heterogeneous DERs in VPPs, also called the polytope model or the flexibility envelope model. A coordination transformation is applied to eliminate redundant variable dimensions while preserving the interface characteristics of the DERs. Furthermore, a sample-based projection method is developed to remove all state variables, resulting in a unified representation of the flexibility region. This method is then employed to compute the Minkowski sums of individual flexibility polytopes, enabling effective aggregation.