[lammps-users] Fix Boundary

Hey all,
In lammps example directory, crack, the file in.crack gives
fix 2 all boundary setforce 0 0 0 0 0 0;
How come I did not find this fix style on the website ?

Because the line in the file is:
fix 2 boundary setforce NULL 0.0 0.0
and its doc/fix_setforce.html