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In Sabrina and MCNP, a surface is described by a general quadratic equation, involving the Cartesian coordinates and coefficients, of the form f(x,y,z) = 0. In surface mode, the + and - operators denote the sense of points with respect to each surface, that is, on which side of the surface the points lie. The sense of a point is the sign of f that results when the coordinates of the point are substituted into the surface equation. If, for example, we have defined planes 1, 2, and 3 so that points with a positive sense lie to the right of the surfaces, the cell description 1 -3 2 means that this cell lies to the right of surfaces 1 and 2 and to the left of surface 3.
The sense with respect to a surface depends on the coefficients used to describe the individual surface. Planes 5 and 6 could be defined so that a point to the right of both has a positive sense with respect to plane 5 and a negative sense with respect to 6.