What means this "Documentation" as opposed to "Registration" ?

Can somebody please explain what these things are?

A document is similar to a certificate of title, (in a very broad sense), but
it is issued by the US Govt. rather than by a state government. It identifies
the vessel and is considered proof of ownership.

Registration is required by most states. It's a fee that is charged to boat
owners for keeping a vessel within the state, regardless of the state or
country where the title or document was issued. (Most states will allow several
months usage without registration to avoid discouraging cruisers and folks
wanting to patronize a repair yard). Registration is usually collected on an
annual basis. It can run from $10 to several thousand, depending on the vessel
and the state.

Most states require boats that have a state title to display a state ID number,
usually well forward on the hull. States cannot require a documented boat to
display ID numbers, but they can require the boat owner to affix a registration
sticker to the hull.