The hammer-on and pull-off elements are used in guitar and fretted instrument notation. Since a single slur can be marked over many notes, the hammer-on and pull-off elements are separate so the individual pair of notes can be specified. The element content can be used to specify how the hammer-on or pull-off should be notated. An empty element leaves this choice up to the application.
The
<pull-off>element.The hammer-on and pull-off elements are used in guitar and fretted instrument notation. Since a single slur can be marked over many notes, the hammer-on and pull-off elements are separate so the individual pair of notes can be specified. The element content can be used to specify how the hammer-on or pull-off should be notated. An empty element leaves this choice up to the application.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/pull-off/