Suppose if one creates a dislocation in a system with two surfaces top and bottom on z axis and periodic in x and y axes. Motions of atoms at the top and bottom layer should be limited along x and y direction and not to move z direction. One can achieve this constraint in LAMMPS by writing as follows, :

region top block      0.0  300.0  0.0  20.0  212.643446964  300.0
region bottom block   0.0  300.0  0.0  20.0  0.0    7.0
group  topatoms  region top
group  botatoms  region bottom
fix  constraint  topatoms  setforce  NULL  NULL  0.0
fix  constraint  botatoms  setforce  NULL  NULL  0.0
velocity all set  0.0  0.0  0.0

Here two regions are defined, called top and bottom. Then groups of atoms called topatoms and botatoms are defined, and the forces of z component are reset to 0.