basically is the order you give to describe a triangle, if you give a counter clockwise order the normal will point in a ''positive'' direction respect the two vectors a-b,b-c, but this is up to you, you can determine if your triangle is counterclockwise or not doing a 4x4 matrix determinant , but you have to specificly give a point as a ''reference'' on the plane.
normally opengl has ccw ( counterclockwise ) as default
someone may correct me because i''m not very well recalling the default value.
sorry for my english