Ferdi adds the option to change the server the application will communicate with to store preferences and such. By default, this is
https://api.franzinfra.com - the official Franz server. This allows Ferdi to stay compatible with your current Franz account.
If you want to experience all Ferdi features, you may want to use a custom ferdi-server or ferdi-slim-server. ferdi-server and ferdi-slim-server allows you to use Premium features without restrictions and adds the ability to package and add additional recipes.
ferdi-server is a full, standalone server - including its own user management system. You can also import your existing Franz account into your ferdi-server to start right where you left off. A custom ferdi-server allows you to sync custom Ferdi settings, such as a custom Todo server or auto-hiding the menubar between installs. This is not the case with the official Franz server.
ferdi-slim-server is only a wrapper around the Franz API. This way you'll still use your regular Franz account and all changes can sync to normal Franz clients. The only difference is, that ferdi-slim-server allows you to add custom service recipes that are available through the recipe store on all connected clients.
More information on how to set up a ferdi-server can be found at https://github.com/vantezzen/ferdi-server/blob/master/README.md.