why vtkResliceCursor set a negative in vtkResliceCursor.cxx file?

when I search the file ‘vtkResliceCursor.cxx’ and find as below:
vtkResliceCursor::vtkResliceCursor()
{


this->ReslicePlanes->GetItem(0)->SetNormal(1,0,0);
this->ReslicePlanes->GetItem(1)->SetNormal(0,-1,0);
this->ReslicePlanes->GetItem(2)->SetNormal(0,0,1);

}
from this function , the x and z axis have set the positive value, why y axis set a negative value? does that mean something special use?