Page vs Subpage
In jumpchart terminology, when you create a “page,” you’re creating a page on the same level as the one you’re currently viewing. If you create a “sub-page” it will be one level lower than the page you are currently viewing.
Remember, if you make a mistake, or if it’s simply quicker, you can add a page anywhere in the site, and later drag it to it’s appropriate spot in the hierarchy using the reorder function. More information on that here.