OpenID logoI’ve defined an Authentication Method Reference (AMR) value called “pop” to indicate that Proof-of-possession of a key was performed. Unlike the existing “hwk” (hardware key) and “swk” (software key) methods, it is intentionally unspecified whether the proof-of-possession key is hardware-secured or software-secured. Among other use cases, this AMR method is applicable whenever a WebAuthn or FIDO authenticator are used.

The specification is available at these locations:

Thanks to Christiaan Brand for suggesting this.