By default, fasm produces System V binaries for ELF64:



This is the same as:



However, I've seen people create Linux binaries too:



What's the difference between the two and which one should I use?

SVR4 is where ELF originated from. So the System V ABI is seen as some sort of a default ABI. If an executable doesn't contain OS-specific extension, the compiler *usually* defaults on 0.

Check Wikipedia ("It is often set to 0 regardless of the target platform.") and also SCO's System V ABI docs ("If the object file does not use any extensions, it is recommended that this byte be set to 0.")

System V is a common ABI on Linux (even when compiling with gcc AFAIK).

On FreeBSD, fasm also sets it to 0. On the other hand, Clang produces FreeBSD (9) ABI objects by default.