There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success), to conform with the C/C++ standard.

This interface has been deprecated and is retained only for compatibility. Its use is discouraged.

BSD-derived OS's have defined an extensive set of preferred interpretations: Meanings for 15 status codes 64 through 78 are defined in sysexits.h.[15] These historically derive from sendmail and other message transfer agents, but they have since found use in many other programs.[16] It has been deprecated and its use is discouraged.

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.