This document provides a guide for the development of a functional specification for Distributed Energy Resources (DER) management systems (DERMS). It includes guiding principles for the application and deployment of DERMS and DERMS control systems, addresses the basic functional requirements and proposes a set of core functions. These include: DER discovery and visualization, and monitoring of real and reactive power flows and voltage at specific nodes; DER production estimation and scheduling, and dispatch of real and reactive power; DER ancillary services provision, including voltage and frequency control/support. The approach is extended to Virtual Power Plant (VPP) control systems.