PUN Classic (v1), PUN 2 and Bolt are in maintenance mode. PUN 2 will support Unity 2019 to 2022, but no new features will be added. Of course all your PUN & Bolt projects will continue to work and run with the known performance in the future. For any upcoming or new projects: please switch to Photon Fusion or Quantum.
Serialization in Photon
Photon and its clients are using a highly optimized binary protocol to communicate. It's compact, yet easy to parse.
Photon must convert all data into this binary protocol before it can be sent. This is done automatically for a range of commonly used data types. On most client APIs, you can also register your custom serialization methods for other classes you might need. See below.