The mechanism for generating public key credentials, as well as requesting and generating Authentication assertions, can be extended to suit particular use cases. Each case is addressed by defining a registration extension.
Standard extensions are usually listed in the dedicated IANA Registry available at https://www.iana.org/assignments/webauthn/webauthn.xhtml
This library is ready to handle extension inputs and outputs, but no concrete implementations are provided, except for the appip extension.
It is up to you, depending on the extensions you want to support, to create the extension handlers.