If anyone else is interested, the specific code can be found here:

http://www.phila.gov/li/UpdDocs/PropertyMaintenanceC.pdf

Here is the pertinent info:
SECTION PM-202.0 GENERAL DEFINITIONS
Building: Any structure occupied or intended for supporting or sheltering any occupancy.
Blighting Influence: A vacant building that lacks windows with frames and glazing and/or lacks one or more doors in entryways of the building if:
1.the building is located on a block face where 80% or more buildings on the block face are occupied; or
2.the Department has provided 20 days notice to the owner of the property that the Commissioner of Licenses and Inspections has determined, in consultation with other City officials as appropriate, that the lack of windows and/or entry doors has a significant adverse influence on the community based on the following factors:
a.deterioration and/or safety of the property;
b.safety of the surrounding community;
c.The value of intact, occupied properties in the surrounding vicinity of the property;
d.marketability of the property; and
e.community morale.