An extent specifies temporary alteration of the display appearance of a part of a buffer. It contains markers delimiting a range of the buffer, plus a property list (a list whose elements are alternating property names and values). Extents are used to present parts of the buffer temporarily in a different display style. They have no read syntax, and print in hash notation, giving the buffer name and range of positions.
See section Extents, for how to create and use extents.