If the unit cell is invariant under certain symmetry operations these can be specified in the .cell file
Each symmetry operation occupies four lines. The first three lines make a 3x3 array that represent a symmetry rotation and
the next line is the translation associated with this rotation.
Another way is to allow CASTEP to automatically find the highest symmetry group that applies to the structure.
Note of caution; with a non-cubic cell or with a cell in which the c vector is not along the z direction the automatic symmetry generation can sometimes fail to find all of the symmetry operations that apply to a supercell.