All styles which exist within your document are shown in the style list.
All documents have several styles to start with, before you start editing and creating additional styles. These are the styles defined in the template from which the document was created.
Documents based on a Letter template will have a base letter style, which is the style in which all text appears, and a header/footer style.
Documents based on a Sheet template will have a base style, a header and footer style, and a current cell style.
Documents based on your own templates will contain any additional styles you have defined for those templates.