Xiandong,
It defines entries in a shape matrix, as defined by Parrinello and Rahman in for instance J. Appl. Phys. 1982, 52, 7182. Shape matrices are based on crystallographic vectors a, b, and c, where h = {a, b, c}. We adhere to the convention a = (xx, 0, 0)^t, b = (yx, yy, 0)^t, and c = (zx, zy, zz)^t, which gives for the shape matrix h
xx yx zx
0 yy zy
0 0 zz
This can describe any crystallographic geometry. In the example below I used
xx = 5 - (-5) = 10
yx = 1
yy = 5 - (-5) = 10
zx = 0
zy = 0
zz = 5 - (-5) = 10
To get LAMMPS to understand this geometry, you will need to add a line currently containing xy xz yz in your data file as outlined in the example below.
Pieter