[cflee]

They continue doing the key wrapping with HMAC for U2F. Resident keys are for the "passwordless" authentication method under FIDO2.

U2F requires that the server must know exactly which keyHandles to request, based on the username (and probably password) that is supplied earlier by the user, so that the token can take the keyHandle and derive the key.

In FIDO2 "passwordless" mode, there's no username or other identifier presented, so it's just a generic request for credential from the server -- the authenticator has to independently figure out which key to present based only on the origin/domain, and maybe even present a list of stored keys (probably effectively a list of accounts?) to the user for selection. So it'd need some local/resident storage of various bits like the origin, maybe a user-chosen account name, and the actual credential, since it can no longer rely on the server to do store all these bits.