A valid domain string that identifies the
WebAuthn Relying Party on whose behalf a given registration or authentication ceremony is being performed.
User
You get the user ID back when checking registration (as userHandle), if you're using client-side discoverable credentials.
You can identify with this ID the user who wants to login.
A user handle is an opaque byte sequence with a maximum size of 64 bytes, and is not meant to be displayed to the user.
The user handle MUST NOT contain personally identifying information about the user, such as a username or e-mail address.
only for display, i.e., aiding the user in determining the difference between user accounts with similar display names.
A human-palatable name for the user account, intended only for display.