Does it matter where you put the tempo marking? For example, above the treble staff, in between the treble staff and the bass staff or underneath the bass staff? I know that when you put a dynamic marking above the treble staff, it only applies to the treble staff, and when you put the dynamic marking in between the treble staff and the bass staff, it applies to both and when you put it underneath the bass clef, it applies only to the bass clef. But this isn't true for the tempo marking right? I believe the tempo marking applies to both staffs always, and it doesn't matter where you put it. Is it true? Or is the tempo marking always above the treble staff?
If there are more instruments (for example in the orchestral score), there may be several identical tempo markings for easier reading, but it's not necessary at all. The only thing that is necessary is to put a tempo indication at the top.
As you see, the tempo indication "Largo" is only at the top and above the string section.
Anyway: wherever the tempo markings are, they of course affect everything. In the example above, everybody plays Largo, not only flutes and first violins. (If you think a little bit about it: the music in all staves must go equally fast. Otherwise you would be on different place in each staff! Your left hand could be at the start of the line and your right hand at the end. That would be most confusing and useless.)
However, they still apply to the whole music.