In addition to the variables described above, there is the exceptional case of a “surface variable” which is neither a weather variable nor a room variable. Currently, the only example of this is the solar irradiance incident on a surface, denoted ii.
It is similar to a room variable in that its evaluation is context dependent; the ii variable is evaluated in reference to the surface to which the formula is attached. Unlike some of the room variables, it has no corresponding function, the evaluation of which would yield the value of the variable in reference to another specified surface (as described in section 12.9 Formula Functions). Therefore, the expression “ii(1)”, for instance, is always illegal.
Note that ii is currently only used in glazing electrochromic control functions and in shading device operation profiles (see the ApCDB help.) It should not be used in any other context.
