Results for surface elements are displayed in relation to their local coordinate system, which is compatible with their Span Direction (see Span Direction). The orientation of Span Direction is transferred to the analytical model and is the basis for defining the X axis of the local coordinate system. The Y axis lies in the plane of a surface element and is perpendicular to the X axis. The Z axis is normal to the element surface.
The sign convention for a surface element. 
In this image the X axis is compatible with Span Direction. Mxx is a bending moment acting in the direction of axis defined by Span Direction (and compatible with the X axis), while Myy is a moment in the perpendicular direction.
Note: To display Span Direction, select the check box at the bottom of the Results dialog. See Visualization of Analysis Results.