stressMask

Hi!
I want to know the stressMask 's meanning in Peri3dController.
in your official document, it sais:

stressMask(=0, all strains)¶

    mask determining whether components of goal are strain (0) or stress (1). The order is 00,11,22,12,02,01 from the least significant bit. (e.g. 0b000011 is stress 00 and stress 11).

can u explain what is it mean?i cant understand it.

 if i use the order
label.stressMask=0b000011

or if i use
label.stressMask=0b100001
what does it mean?
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you can use the following code:
#!/usr/bin/python # This is server.py file
import string
from yade import plot,qt
from yade.pack import *
from yade import pack, plot
import socket # Import socket module

O.materials.append(CpmMat(damLaw=0,young=170e9,density=4800,frictionAngle=atan(0.8),poisson=0.4,sigmaT=20e8,epsCrackOnset=0.8e-4,relDuctility=300))
initSize=0.028

sp=pack.randomPeriPack(radius=.000625,initSize=Vector3(initSize,initSize,initSize))
O.periodic=True
sp.toSimulation()

O.save('/tmp/cccc.gz')

O.dt=1e-7

plot.plots={'ex':('sx',)}
def plotAddData():
 plot.addData(
  sx=p3d.stress[1],
  ex=p3d.strain[1],
 )

EnlargeFactor=1.5
EnlargeFactor=1.0
O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(aabbEnlargeFactor=EnlargeFactor,label='bo1s')]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(interactionDetectionFactor=EnlargeFactor,label='ig2ss')],
  [Ip2_CpmMat_CpmMat_CpmPhys()],[Law2_ScGeom_CpmPhys_Cpm()]),
 NewtonIntegrator(),
 Peri3dController(

       nSteps=10000, # how many time steps the simulation will last
       # after reaching nSteps do doneHook action
       doneHook='print "Simulation with Peri3dController finished."; O.pause()',

       label='p3d'
       ),
 PyRunner(command='plotAddData()',iterPeriod=1),
]

p3d.stressMask=0b100001 # prescribed ex,ey,sz,syz,ezx,sxy; e..strain; s..stress
p3d.goal=(0,-0.008,0,0,0,0) #xx, yy, zz, yz, zx, xy

O.step()
bo1s.aabbEnlargeFactor=ig2ss.interactionDetectionFactor=1.
O.run(); O.wait()
plot.saveDataTxt('biaozhun.txt')
plot.plot(subPlots=False)

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the model is used to add strain on the model till the stain is [0,-0.008,0,0,0,0],so what is the use of stressMask?
if i delete "p3d.stressMask=0b100001 ", what will it happen?