The raised 15th is only used when the 9th in a chord is natural. It functions as a minor ninth, creating a major seventh interval with the natural 9th, assuming that the chord is in root position. The notation of a raised 15th is a fairly modern addition to Western harmony, and they have been popularized by contemporary musicians like Jacob Collier. Natural 15ths are never notated as alterations or extensions, as they are enharmonically equivalent to the root. For example, a chord that includes a raised fifteenth could look something like G13(515).