When you select Automatic in text formatting dialogs, the format attributes of a higher or parent style pass through to the lower or child style.
The order of the cascading styles is:
The SiteStyle is the parent of the Site, which is the parent of the Page. Thus, selecting Automatic for an attribute at the Page level lets the setting for that attribute pass from the Site to the Page. If the same attribute is set to Automatic in the Site as well, then the SiteStyle setting for that attribute flows to the Site and then, unimpeded, to the Page.
For example, if you set the font color to red at the SiteStyle level and set font color to Automatic on both the Site level and the Page level, text at the page level is automatically red when you type it. The red attribute flows all the way from SiteStyle to the text.
When you change a setting at a higher level, it changes the corresponding setting at the lower level if the setting at the lower level is Automatic. So, if the setting at the Page level is Automatic, and you change the font at the Site level, the font setting changes for the Page level, too. This feature provides you with a fast, powerful way to make formatting changes throughout your site by simply making the change once at the higher levels.
Of course, there may be times when you don’t want the changes from the higher levels to be inherited by the lower levels. In that case you can edit the text style so Automatic is not selected for the particular format attribute. Editing a style at any level overrides the settings from the parent level. For example, if you select text and change its color to blue, that editing overrides a setting of red text from the SiteStyle level.
You can mix choices among the formatting attributes so some have the Automatic setting and others don’t. For example, you could select the font to be inherited from a higher level (so you set it to Automatic), but manually select the color.
See Working with Text Styles for more information about the flow of text styles and formats to text in your site.