The style selector in the Ribbon Toolbar functions as an indicator to identify the style of the current text or paragraph. It displays the paragraph and/or text style at the current cursor position or of the currently selected text. For example, in the screenshot below, the cursor is in a paragraph formatted with a style called Body Text:
If you manage multiple projects that all need to look the same, check out Help+Manual's powerful style repositories. They allow you to share stylesheets between multiple projects and edit the styles for all of them in the same central location. See the Working with Style Repositories chapter for details!
Dynamic Styles (Reference)