dynamic with staff omitted

[rpatters1]

The documentation for dynamic.staff states:

The staff index to which this dynamic applies. By default, a dynamic applies to all staffs in the part.

I would understand this if we were speaking about playback. But for creating a score it leaves me in doubt. Does it mean that the dynamic is to appear on every staff of the part? This would not be typical of keyboard music, for example. There, a dynamic placed between the staves semantically applies to both staves.

It seems like this comment is trying to achieve an awful lot. I don't fully understand it.