The methods startSheet() and startCell() in CellHandler is to inform client application that LightCells processor encounters one new sheet/cell, that is, this method will be invoked by LightCells for every sheet/cell. If client application wants to process this sheet/cell, then this method should return true, so LightCells will parse data for this cell and invoke method process() on it; Otherwise return false, then LightCells processor will ignore this sheet/cell and move to next sheet/cell.
To process part of all cells, I think you can implement your startCell() method of CellHandler as following:
private int startRow, endRow, startColumn, endColumn;
public boolean startCell(int row, int column)
if(row >= startRow && row <= endRow && column >= startColumn && column <= endColumn)