Stiffeners are defined as beam elements, but they are not actual finite elements in a coarse mesh model.  Instead, they are treated as additional stiffness in the defined direction for the panel element they are defined on, thus converting the material to orthotropic.  In a coarse mesh model, all stiffeners are treated as "internal" regardless of their defined location.  The number of stiffeners, whether defined as internal or edge, is the key to how the mechanical properties of the panel element are changed.  However, the location is relevant when creating a fine mesh model from the coarse mesh model.  At this point, an actual beam element will be created representing the properties of the stiffener at the defined location