Decarbonization requires the integration of many small decentralized energy resources. In particular, these decentralized energy resources have to provide flexibility to the energy system. In this paper, we suggest a cellular approach to describe the energy system and propose mechanisms for the exchange of flexibility in a cellular energy system. For this purpose, we break down the exchange of flexibility into four dimensions: The flexibility offer, the flexibility retrieval signal, the transmission path of the retrieval signal, and the responsibility for the execution of the retrieval signal. The paper discusses possible specificities of these dimensions and explains how these can be combined into different mechanisms to integrate the flexibility of decentralized energy resources.