From the NATO Topic page on Centres of Excellence:
Centres of Excellence (COEs) are nationally or multi-nationally funded institutions that train and educate leaders and specialists from NATO member and partner countries, assist in doctrine development, identify lessons learned, improve interoperability, and capabilities and test and validate concepts through experimentation. They offer recognized expertise and experience that is of benefit to the Alliance and support the transformation of NATO, while avoiding the duplication of assets, resources and capabilities already present within the NATO command structure.
Coordinated by Allied Command Transformation (ACT) in Norfolk, Virginia in the United States, COEs are considered to be international military organizations. Although not part of the NATO command structure, they are part of a wider framework supporting NATO Command Arrangements. Designed to complement the Alliance’s current resources, COEs cover a wide variety of areas, with each one focusing on a specific field of expertise to enhance NATO capabilities.