Many times I've been asked by students about the optional keys on the flute, and their pros and cons. Since many companies use different names for the same key, deciding what optional keys to choose when ordering a flute can be pretty daunting.
Here is a list of the most common optional keys for the flute and some less common ones, as well.
In the first movement of Pini di Roma, "I pini di Villa Borghese", Ottorino Respighi writes an F#A3 tremolo in the first flute part.
This tremolo is awkward to play with regular fingerings, and hard to sound with harmonic fingerings (F#1 & A1) without any modifications.