Play Transposition

add a transposing Instrument to a score and look at it's staff properties.
I think than it becomes pretty much self explanatory?

Well, it's self-explanatory if you understand the concept of a transposing instrument. If not, then look up that phrase on Wikipedia or elsewhere. Basically, many instruments (including most wind instruments) are designed such that they sound lower than written. Playing a *notated* C on a clarinet, for example, yields a note that *sounds* like a Bb. think of it as like a permanent unchanging capo, if that helps.

So the Play Transposition field defines that relationship. Pressing the Concert Pitch button toggles the score display between the written and sounding pitch for transposing instruments.

As for the Handbook, maybe see the section labeled "Transposition"?