what means "accf" in soisconnected function? thanks!

[Jon]

I am reading 'soisconnected' function in 'Uipc_socket2.c' file.
I find this lines. But I cannot find accf's mean. can you explain to me?thank you very much!
  if ((so->so_options & SO_ACCEPTFILTER) == 0) {
    ...
  } else {
   ACCEPT_UNLOCK();
   SOCK_LOCK(so);
   so->so_upcall =
       head->so_accf->so_accept_filter->accf_callback;
   so->so_upcallarg = head->so_accf->so_accept_filter_arg;
   so->so_rcv.sb_flags |= SB_UPCALL;
   so->so_options &= ~SO_ACCEPTFILTER;
   SOCK_UNLOCK(so);
   so->so_upcall(so, so->so_upcallarg, M_DONTWAIT);
  }