Logical styles tag text according to its meaning, while physical styles indicate the specific appearance of a section. In the ideal SGML universe, content is divorced from presentation. Indeed many browsers today let you define how you want the various HTML tags rendered on-screen. Another advantage of logical tags is that they help enforce consistency in your documents.
Most browsers render it in bold text.
However, it is possible that a reader would prefer that these sections be displayed in red instead. Logical styles offer this flexibility. Physical styles, therefore, offer consistency in that something you tag a certain way will always be displayed that way for readers of your document. Try to be consistent about which type of style you use. If you tag with physical styles, do so throughout a document.