If you are looking for specific guidance on managing your license or getting started, the following sections from the official EAGLE 5.6.0 Help are most relevant:
In version 5.6.0, the eagle.key file is the central license component. Eagle 5.6.0 License.key
Highly reliable for creating complex multi-layer boards. If you are looking for specific guidance on
with modern Eagle versions, nor with Fusion 360. nor with Fusion 360.