If I see in piano sheet music, say, "mp" mark between bass and treble staves, which staff should be played in "mp" dynamics?
Reverse question: if I want to apply a dynamic mark to a specified staff, how to notate this?
On piano music, with treble and bass clefs, if the dynamics mark is between them, it refers to both parts (hands). If it's for the treble, it's found above the treble, and if for bass alone, it's found under the bass.
Just to add some working example to the thread, here goes Prokofiev op. 97, no. 10. An adagio from the Cinderella suite for piano. The complete score for this adagio is available here, page 29.
In the first two bars, there are specifications for both staves (forte), and for the upper (piano) and lower (mezzoforte) staves only.
In scores for greater ensembles as well as for instrument groups (say 2 bassoons and contrabassoon notated in the same score) the dynamic is typically written below the voice it belongs to.