A given element may have only one of the Horizontal, Vertical or Slope constraints assigned to it. If you attempt to assign a Vertical constraint to an element that is already assigned a Horizontal constraint, the Vertical constraint will replace it.

The action taken when you assign a Vertical constraint to an element that is already assigned a Slope constraint depends on whether or not the Slope constraint is parameterized. If it is not, then the Vertical constraint simply replaces it. If the Slope constraint is parameterized, however, the Vertical constraint is not assigned. Instead, the value of the parameter is adjusted to 90° or 270° (effectively vertical). This is done so as not to invalidate any expressions that may reference the existing slope parameter.