When the music on a page exceeds the page fill threshold, MuseScore will try to space the music vertically on the page (Justify vertically). If not the music is aligned at the top of the page.
I like to keep it at 100% while fiddling with spacing, because otherwise, MuseScore may stretch out pages that exceed the threshold and you won't see the effect of your spacing changes. But once I'm happy with the spacing overall, I'll set the threshold to something lower, maybe 70-90%. That eliminates the extra space that might otherwise exist at the bottom of some pages where it almost but couldn't quite fit in another system.
Comments
When the music on a page exceeds the page fill threshold, MuseScore will try to space the music vertically on the page (Justify vertically). If not the music is aligned at the top of the page.
I like to keep it at 100% while fiddling with spacing, because otherwise, MuseScore may stretch out pages that exceed the threshold and you won't see the effect of your spacing changes. But once I'm happy with the spacing overall, I'll set the threshold to something lower, maybe 70-90%. That eliminates the extra space that might otherwise exist at the bottom of some pages where it almost but couldn't quite fit in another system.