What is a “Finish Rating” found on some fire-rated assemblies?

A finish rating is established for assemblies containing combustible (wood) structural supports. The finish rating is defined as the time at which the wood stud or wood joist reaches an average temperature rise of 250°F or an individual temperature rise of 325°F as measured on the plane of the wood nearest the fire during the fire test using the standard time-temperature curve. The temperature reading is measured between the gypsum and the stud.