The term characteristic length or scale refers, in general, to the parameter which characterizes the density profile (of a given physical quantity). The static (equilibrium) or dynamic character of a characteristic length must be specified. The terms out of plane and in plane refer to characteristic lengths normal or parallel to the @I03082@, respectively. Since @I03085@ 'thickness' and characteristic length correspond to various concepts, the current usage where an out of plane characteristic length is referred to as the @I03085@ thickness, is confusing and should be abandoned.