Thematic Break (<hr>)

When To Use 

• Denotes a thematic break at the paragraph level, such as a shift in the setting of a narrative or a shift to a different subject within a section of a scholarly work. ¹

Rules

• It is intended to differentiate paragraphs or bocks of text from another. If another element already groups these blocks together (like a <section> element), then an <hr> element is not semantically needed. ¹

Sources 

1. HTML Spec: Living Standard, 4.4 Grouping content, The hr element is licensed under Creative Commons Attribution 4.0 International License