(Disclaimer: First of all, I don't own Pokemon or any of the characters. Now that that's done with...)

Choices are the following:
1: Start with a girl from Kanto.
2: Start with a girl from Johto.
3: Start with a girl from Hoenn.
4: Start with a girl from Sinnoh.
5: Catch-all Misc.

There is no reason to think that you need to stay within a region. For example, Dawn could go to all three of the other regions and so on. Anime specific plots and characters are also allowed regardless of section. Your choice only determines the main character of the story.