Margins, page sizes, and distances are all measured in tenths to keep MusicXML data in a consistent coordinate system as much as possible. The translation to absolute units is done with the scaling type, which specifies how many millimeters are equal to how many tenths. For a staff height of 7 mm, millimeters would be set to 7 while tenths is set to 40. The ability to set a formula rather than a single scaling factor helps avoid roundoff errors.
The
<scaling>element.Margins, page sizes, and distances are all measured in tenths to keep MusicXML data in a consistent coordinate system as much as possible. The translation to absolute units is done with the scaling type, which specifies how many millimeters are equal to how many tenths. For a staff height of 7 mm, millimeters would be set to 7 while tenths is set to 40. The ability to set a formula rather than a single scaling factor helps avoid roundoff errors.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/scaling/