Final answer:
A subheading is the typographical feature that typically indicates a major shift in focus in written materials, segregating content into different sections or topics.
Step-by-step explanation:
The typographical feature that usually announces a major shift in focus in written materials is a subheading. Subheadings serve as a tool to segment the content into different sections, often indicating a change or new topic within the chapter or section of a book, article, or report. In contrast, photo captions describe images, boldface can highlight important terms or headings, and italicized words are used for emphasis or to denote titles of certain works.