Clarity about extern(Windows)/extern(System)

[Kagamin]

 From https://gcc.godbolt.org/

__attribute__((stdcall))
int square(int num) {
     return num * num;
}

_Z6squarei:
   push ebp
   mov ebp, esp
   mov eax, DWORD PTR [ebp+8]
   imul eax, DWORD PTR [ebp+8]
   pop ebp
   ret 4