See Fact Sheet on the Housing Environmental website: https://files.hudexchange.info/resources/documents/Acceptable-Separation-Distance-Fall-Hazards-and-Pipelines-Fact-Sheet.pdf
Here is the relevant content from the Fact Sheet:
How does HUD define the terms ancillary facilities and common areas as they apply to fall hazards and pipelines analysis?
The restrictions on fall hazards and pipelines apply to buildings, ancillary facilities, structures, or common areas. HUD defines ancillary facilities as facilities ancillary to housing. This would include onsite areas that are not residential buildings, but are there to support or complement residential buildings and are used by people, including private balconies, front or back yards, divided green space or patios, carports, garages, sheds and pergolas or buildings like gyms, pool houses, etc. Ancillary facilities would also include common areas, which are the non-private ancillary spaces on the housing site that residents are allowed or expected to access. Common areas are areas where people would likely congregate, and include playgrounds, and outdoor recreation areas. Parking lots included within the site boundaries of, and designed to serve the residents of, a HUD-insured residential or mixed-use development are covered under this definition.
Walking trails, pathways, and sidewalks that include items such as sitting benches, tables, pergolas, or gazebos are always considered common areas of congregation. Walking trails, pathways, and sidewalks without these amenities may be excluded on a case-by-case basis (for example, a trail in a natural area that is not actively managed or monitored by the property, or a trail segment that connects to a larger biking or walking trail network.)