[dev] about sw\source\core\text\itrcrsr.cxx\SwTxtMargin::nLeft

Hi :
I have a question ,           

in sw\source\core\text\itrcrsr.cxx\SwTxtMargin::CtorInitTxtMargin( SwTxtFrm 
*pNewFrm, SwTxtSizeInfo *pNewInf )
{ 
.......
 nLeft = pFrm->Frm().Left() +
                    pFrm->Prt().Left() +
                    nLMWithNum -
                    pNode->GetLeftMarginWithNum( sal_False ) -
                    rSpace.GetLeft() +
                    rSpace.GetTxtLeft();
......
}

who can tell me the meaning the expression on the right of "=" associated with 
the frame?