The National Dex Edit
The National Dex contains every single pokemon to it's version.
So if the game is in Sinnoh all pokemon to sinnoh is in the pokedex, for excample from bulbasaur to arceus.
depending on what region all the pokemon to that region are in the National Dex, but it dosen't come with all the pokemon once you get it you have to fill it up with all pokemon.
In pokemon HG & SS the national dex goes all the way to sinnoh
Did you Know?.......Edit
.....In Sinnoh you need the National Dex to regesture some legends that are from Sinnoh like heatran and regigigas.