In the present context it is not necessary to explain all the Space Group symbols, but, for the aficionado here they are :
The (sub)symbol 21 means a 2-fold screw axis.
m is a mirror plane.
a is a glide plane with translation component parallel to the a-axis.
n is a glide plane with translation component in a diagonal direction.
P means that the unit cell has points (which can be occupied by motifs) only at its corners.