CloudCompare源码中DgmOctree不理解的地方

在查看CloudCompare源码过程中，有一个地方不理解：

OctreeCellCodeType predCode = (p->theCode >> bitDec);  //这里theCode左移操作怎么理解呢？谢谢啊~

//---------------------------------------------------------
此变量出自：
类 class DgmOctree
成员：    
 cellsContainer m_thePointsAndTheirCellCodes;

//! Container of 'IndexAndCode' structures
#define unsigned long long OctreeCellCodeType;
typedef std::vector cellsContainer;
struct IndexAndCode
{
        //! index
        unsigned theIndex;
        //! cell code
        OctreeCellCodeType theCode;
……
}

//-------------------对theCode赋值是这的--------------//
it->theCode = generateTruncatedCellCode(cellPos,MAX_OCTREE_LEVEL);

generateTruncatedCellCode（）
{
    const unsigned char dec = MAX_OCTREE_LEVEL-level;
    ( PRE_COMPUTED_POS_CODES.values[cellPos.x << dec]
| (PRE_COMPUTED_POS_CODES.values[cellPos.y << dec] << 1)
| (PRE_COMPUTED_POS_CODES.values[cellPos.z << dec] << 2)
) >> GET_BIT_SHIFT(level);
}


//---------------------对theCode是这样使用的---------------//
//binary shift for cell code truncation
unsigned char bitDec = GET_BIT_SHIFT(level);
//#define GET_BIT_SHIFT(level) (3*(CCLib::DgmOctree::MAX_OCTREE_LEVEL-level));
//iterator on octree elements
cellsContainer::const_iterator p = m_thePointsAndTheirCellCodes.begin();
//we init scan with first element
OctreeCellCodeType predCode = (p->theCode >> bitDec);  //这里theCode左移操作是什么作用呢？谢谢啊~