A question about creating a system call

[dave jones]

Hello,

I know how to create system calls, but I'm a bit confused about
sys/kern/syscalls.master file explained. For example, if I have a
foo system call, following code is added:

532     AUE_NULL        STD     { int foo(char *str); }

The question is in column two AUE_NULL, can I replace it with AUE_FOO?
How to determine the system call should be audit or not? Thank you.

Regards,
Dave.