Using the Roon Remote app on my iPhone 6S to control my library core on my PC. Is there a way to transfer the audio between zones using the iPhone like I can do with the main interface on my PC?
Thanks.
Yes, but its not obvious or where you think it might be 
Press the volume to select the zone you want to be active on the phone. Then press on the bottom bar to see ‘now playing’, and at the top left of that screen is an icon with two arrows, press that to xfer. Just don’t ask me why, it comes from a mind wired very differently to mine (no offence to anyone).
[you found it anyway, well done]
Never mind. I just saw the opposing arrows icon top left of the album art of Now Playing.
Sorry – I should have put my reading glasses on first. 
Thanks, Steve. We cross-posted.
I too went around a few times to finally find it…only so much you can have on a screen based on its size at one time.