sth about stressmask

Hello,
     The following code is an example from Yade. I know the first part is about hydrostatic compression. But how to understand "stressMask=0b000111" here? ( goal=(-1e7,-1e7,-1e7, 0,0,0) ) As we know, 0 means strain controlled and 1 means stress controlled, I think stressMask may be set as 0b111111. Could you please give me some suggestions? Thanks.

# peri3dController_triaxialCompression.py
# script, that explains funcionality and input parameters of Peri3dController on the example of
# triaxial compression.
# Firstly, a hydrostatic preassure is applied, then a strain along z axis is increasing
# while x- and y- stress is constant
# The simulation is run on rotated cell to enable localization and strain softening
# (you can also try simulation with command sp.toSimulation() with no rotation,
# in this case there is almost no difference, but in script peri3dController_shear,
# the cell rotation has significant effect)

from yade import pack,plot,qt

# define material
O.materials.append(FrictMat())

# create periodic assembly of particles
initSize=1.2
sp=pack.randomPeriPack(radius=.05,initSize=Vector3(initSize,initSize,initSize),memoizeDb='/tmp/packDb.sqlite')
angle=0
rot=Matrix3(cos(angle),0,-sin(angle), 0,1,0, sin(angle),0,cos(angle))
sp.toSimulation(rot=rot)

# plotting
plot.live=False
plot.plots={'iter':('sx','sy','sz','syz','szx','sxy',),'iter_':('ex','ey','ez','eyz','ezx','exy',),'ez':('sz',)}
def plotAddData():
 plot.addData(
  iter=O.iter,iter_=O.iter,
  sx=p3d.stress[0],sy=p3d.stress[1],sz=p3d.stress[2],
  syz=p3d.stress[3],szx=p3d.stress[4],sxy=p3d.stress[5],
  ex=p3d.strain[0],ey=p3d.strain[1],ez=p3d.strain[2],
  eyz=p3d.strain[3],ezx=p3d.strain[4],exy=p3d.strain[5],
 )

O.dt=PWaveTimeStep()/2

# define the first part of simulation, hydrostatic compression
O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb()]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],[Law2_ScGeom_FrictPhys_CundallStrack()]),
 NewtonIntegrator(),
 Peri3dController( goal=(-1e7,-1e7,-1e7, 0,0,0), # Vector6 of prescribed final values
     stressMask=0b000111,
     nSteps=500,
     doneHook='print "Hydrostatic load reached."; O.pause()',
     youngEstimation=.5e9, # needed, when only nonzero prescribed values are stress
     maxStrain=.5,
     label='p3d'
       ),
 PyRunner(command='plotAddData()',iterPeriod=1),
]
O.run(); O.wait()

# second part, z-axis straining and constant transversal stress
O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb()]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],[Law2_ScGeom_FrictPhys_CundallStrack()]),
 NewtonIntegrator(),
 Peri3dController( goal=(-1e7,-1e7,-4e-1, 0,0,0), # Vector6 of prescribed final values
       stressMask=0b000011,
       nSteps=1000,
   xxPath=[(0,1),(1,1)], # the first (time) zero defines the initial value of stress considered nonzero
   yyPath=[(0,1),(1,1)],
   doneHook='print "Simulation with Peri3dController finished."; O.pause()',
   maxStrain=.5,
   label='p3d',
   strain=p3d.strain, # continue from value reached in previous part
   stressIdeal=Vector6(-1e7,-1e7,0, 0,0,0), # continue from value reached in previous part
       ),
 PyRunner(command='plotAddData()',iterPeriod=1),
]
O.run();O.wait()
plot.plot(subPlots=False)