Here’s a somewhat longer description that draws on the above.
Vertical Justification Settings Primer
(with credit to @benwiggy and @jaskarbong)
Basic Rules
- If the frame fullness is greater than the first threshold, then the frame will be justified by adding space between both staves and systems
- But if the frame fullness is greater than the second threshold, then space will only be added between systems, not staves
- Unless there’s only a single system and the checkbox is ticked, in which case space will be added between staves
Examples
The demo page below has 2 systems, filling c. 60% of the frame.
Example 1
Both settings only activate justification when the frame is completely full; in other words, it will never happen (or will make no difference to an already full frame).
Result: Our 60%-full frame is not justified, and the values in the Minimum Gaps settings are used.
Example 2
Both settings activate justification when the frame is empty; in other words, the frame will always be justified. However, the second option takes priority.
Result: The systems are justified by adding space between the systems. The staves are spaced using the Minimum Gaps settings.
Example 3
The 60%-full frame falls above the threshold for justifying staves and systems (50%), and below the threshold for only justifying systems. So staves and systems are justified.
Result: The music is justified by proportionally increasing space between the staves and systems.
Notes
- Changing the settings to 0%, 100% would mean that all staves and systems would always be justified.
- In Example 2, if the frame only had a single system, then the staves will be justified if the checkbox is ticked.