I think they are trying to specify the depth of the perimeter zones for a model using core and perimeter zoning. Per ASHRAE 90.1, this depth is 15 ft
90.1-2004 Appendix G Table G3.1 Section 8
8. Thermal Blocks ? HVAC Zones Not Designed
Where the HVAC zones and systems have not yet been designed, thermal blocks shall be defined based on similar internal load densities, occupancy, lighting, thermal and space temperature schedules, and in combination with the following guidelines:
(a) Separate thermal blocks shall be assumed for interior and perimeter spaces. Interior spaces shall be those located greater than 15 ft from an exterior wall. Perimeter spaces shall be those located within 15 ft of an exterior wall.
(b) Separate thermal blocks shall be assumed for spaces adjacent to glazed exterior walls; a separate zone shall be provided for each orientation, except that orientations that differ by less than 45 degrees may be considered to be the same orientation. Each zone shall include all floor area that is 15 ft or less from a glazed perimeter wall, except that floor area within 15 ft of glazed perimeter walls
having more than one orientation shall be divided proportionately between zones. (c) Separate thermal blocks shall be assumed for spaces having floors that are in contact with the ground or exposed to ambient
conditions from zones that do not share these features. (d) Separate thermal blocks shall be assumed for spaces having exterior ceiling or roof assemblies from zones that do not share these features.