What are the ssh-keygen -D and -U parameters for? How are they used with smartcards?

The ssh-keygen manpage lists the following two parameters:

-D reader = Download the RSA public key stored in the smartcard in reader.

-U reader = Upload an existing RSA private key into the smartcard in reader.

I can't find out anything else about these after searching around. Do they support a standard ssh_rsa keypair on a smartcard, or do they work with a pkcs11 interface and x509 formatted certificates. Is this related to ssh certificates and able to support storing either the private signing-key or private user-key-with-certificate on a smartcard?

In short, why would I use these parameters?