You can allow/disallow instrument changes in each layout independently; for example, if you want to show multiple percussion instruments on as few staves as possible in the score, but on separate staves for each percussion instrument in the percussion part.
Disallowing instrument changes shows all instrument staves in the selected layouts, including multiple instruments held by one single player.
If you want to input notes for other instruments held by single players but keep instrument changes in the layout, you can switch to galley view to see all staves in the project.
Procedure
Result
Instrument changes are allowed in the selected layouts when Allow instrument changes is activated, and disallowed when it is deactivated.
Multiple instruments can only appear on the same staff with an instrument change if none of their notes overlap. If any of their notes do overlap, multiple staves are shown.