With such a high chapter number I can only think NFPA 101 which indeed has chapter 42.8 addressing parking structures. Although not an expert, I doubt the logic of 20% opening required is applied to any kind of wall piece. I believe the intent is to design the walls in such way to have a fee area of 20% of a virtual separation between building areas. That's probably why it refers to "lines" not "walls".
Most building codes like the ICC mimic the NFPA or vice versa but rear like this:
406.3.3.1 Openings. For natural ventilation purposes, the exterior side of the structure shall have uniformly distributed openings on two or more sides. The area of such openings in exterior walls on a tier must be at least 20 percent of the total perimeter wall area of each tier. The aggregate length of the openings considered to be providing natural ventilation shall constitute a minimum of 40 percent of the perimeter of the tier. Interior walls shall be at least 20 percent open with uniformly distributed openings.
The intent is to create free flow of air thru the parking garage which interior walls would interfere with. Hence the "uniformly distributed openings" clause. This part of the code is vague.
