This object simply reports which physical line on the BNM (broadband network module) the current-day statistics row belongs to, and per the description the range is fixed at 1 because this card only has one line. It exists purely as a table index rather than a real measurement, so an admin will never see it change or use it diagnostically -- it just confirms which row of the CurrentDay table is being read when there is only one possible row. On this card, this object will always read exactly 1, whereas on a multi-line SONET card the equivalent index might range from 1 to 4 or more.