The formula of stress by fix deform

Dear LAMMPS users,

I would to calculate the elasticity by using the fix deform command.The box is uniaxial deformed along x direction. The strain can be obtain by the formula (Lx-l0)/L0, the average tensile
stress σT is obtained from the deviatoric part of the stress tensorσT = (1 + μ)(−Pxx +Σi(Pii/3)), where Σi(Pii/3) is the hydrostatic 
pressure and PT is the normal pressure component in the strain direction. The parameter μ stands for Poisson’s ratio, which is
equal to 0.5 here.[1]
My question is why the σT represent the stress, and in triaxial deformation the σT=-Pxx. [2]
In addition, I am confused with the direction of the conponent of pressure tensor pxx,pyy,pzz,pxy,pxz,pyz. Can someone help me with this questions,it will help a lot.
 Thanks in advance.