Hi, a question about "struct sysentvc"

[mokhi]

Hi guys.
I was reading/studying code of Elf image activator to see how binary
stars on FreeBSD.
On included file (sysent.h), I saw 3 declarations:
extern struct sysentvec aout_sysvec;
extern struct sysentvec elf_freebsd_sysvec;
extern struct sysentvec null_sysvec;

I looked at their definitions too (though i couldn't find
elf_freebsd_sysvec, where's it ?).
I have some (maybe simple, so sorry for it :D) questions.

1) what's sysentvec for, basically ? and in which concept it connects
to image-activators (such as ELF image activator [or a.out's])

2) is there any reference/guide to see how should/can we define a
"struct sysentvec" for new format ?

FWIW, I'm also a little familiar with NetBSD way of binary
running/compat layer devel.

Thanks and thousands of regards, Mokhi.