According to Article 41.3 of the Model Grant Agreement, a consortium agreement has to be signed between beneficiaries, unless the work programme specifies that there is no need for it. In other words, if not indicated otherwise, the beneficiaries must have internal arrangements regarding their operation and co-ordination to ensure that the action is implemented properly (consortium agreement).
The Grant Agreement is a contract set between the consortium (all its partners) and the European Commission, while the Consortium Agreement is an internal arrangement set between the partners of the consortium - it regulates all issues that are not covered by the Grant Agreement but are important for the internal functioning of the consortium (such as the organisation of work, structure, intellectual property management, liability, conflict resolution, future exploitation and dissemination of results, etc.).
In principle, the agreement may include any arrangements the partners wish to make, as long as they are not contrary to the Grant Agreement and the Horizon 2020 Rules for Participation.