If you kill ffmpeg process it will exit with code 255 (easily reproducible in the terminal)

Now this probably won't make difference in the real world (e.g. because the exit codes are not portable and on top of that not always unambiguous as discussed in Default exit code when process is terminated?)

However, because so many shells follow that convention of having 128 + signal_number, programs know to avoid using those values above 128 for their exit code (or when they do exit(130), it's to report the death of a child that dies of a signal 2 like some shells do under some circumstances).

it propagates (Option -p) the returnlevel ($?) so you may use it for your purpose.