Shanjida,
The negative purely indicates that the direction of the power is along the negative direction along the normal of the observation area. In the case of an XY observation area power moving along +z (from left to right in the standard layout window) is positive and power moving along the -z (from right to left) is negative. The charts that show this have an check box which will show the absolute value which is typically what is used in publications provided direction is indicated through the figure caption.
Scott