Continuing from Discussion entry above ...

IOW, it refers to the means of diminishing the risk of fire spreading from one floor to the floor above by means of windows: the total of C + D is the distance flames would have to travel, vertically and horizontally, to reach the glazing of the window above. I suspect it's an entirely French approach. This is one English approx. equivalent:

704.9 Vertical separation of openings. Openings in exterior walls in adjacent stories shall be SEPARATED VERTICALLY TO PROTECT AGAINST FIRE SPREAD on the exterior of the buildings where the openings are within 5 feet (1524 mm) of each other horizontally and the opening in the lower story is not a protected opening with a fire protection rating of not less than 3/4 hour. Such OPENINGS SHALL BE SEPARATED VERTICALLY AT LEAST 3 FEET (914 mm) by spandrel girders, exterior walls or other similar assemblies that have a fire-resistance rating of at least 1 hour OR BY FLAME BARRIERS THAT EXTEND HORIZONTALLY AT LEAST 30 INCHES (762 mm) beyond the exterior wall. Flame barriers shall also have a fire-resistance rating of at least 1 hour. The unexposed surface temperature limitations specified in ASTM E 119 or UL 263 shall not apply to the flame barriers or vertical separation unless otherwise required by the provisions of this code.
http://www.iccsafe.org/cs/codes/Documents/2006-07cycle/Propo...

Here it's vertical distance OR horizontal distance, no mention of combining the two.

Something like "No stipulation regarding French "C+D" vertical and horizontal fire propagation distance between windows, except above patios". Or a footnote explaining.