I'll take a crack at it.
I consider a
Fly Away to be when the Spark enters Atti-Mode for some reason and "flies away" with the wind as the Spark has lost GPS and will not stay in the same position.
The Spark should stay at the same height, but be at the mercy of the wind.
The Spark should still be able to be flown, but needs your input on where you want it to go. I use the map as a quick reference for the Sparks orientation to help fly.
This is why it's good to fly VLOS and maybe practice with a non GPS drone so that you'll really know how to "fly" the Spark during Atti-mode.
When the Spark is in Atti-mode, the RTH won't work because it doesn't know where it's at.
If the GPS is reestablished during the flight, all is well. Unless it drifted with the wind too far for a safe RTH, then it should land somewhere on it's way back when the
battery is critical.
I'll fly into the wind at the start of a flight and use it for a faster flight back.
There have been a few posts that I have read that DJI has warrantied some of the fly aways thanks to the flight logs on the device after review.
RC Signal Lost is when you have a signal loss between the Spark and remote for some reason.
Improper antenna arrangement, flying too far away, Bluetooth, WiFi or trees and buildings in the area blocking the signal.
The Spark still has a GPS lock and the RTH should kick in if the signal hasn't been reestablished.