Which nearby openings matter?
Windows, doors, vents, air intakes and other carbon-monoxide entry points matter, not just the nearest wall. Stationary generators also create carbon-monoxide hazards, so placement is a design task rather than a simple distance check.
Portable-generator spacing cannot be substituted for a stationary model installation manual.
What can obstruct exhaust or access?
Corners, decks, fences, shrubs, HVAC equipment and drainage can affect exhaust, airflow and maintenance access. The site needs enough room not only for the cabinet, but also for the way air moves around it and the way service is performed.
A safe layout is part of the installation design, not an afterthought.
Why one clearance number is not enough
Clearance depends on the specific model, its labeling, openings, airflow, carbon-monoxide pathways and local requirements. A blanket number is not enough because the manual and the authority can change what is acceptable.
Reduced rear spacing can be conditional, but only when the specific installation design and requirements support it.
What a site survey should establish
A site survey should map fuel and electrical routes, the pad or base, drainage, access and the approval path that applies to the property. That is the information needed before anyone treats the location as final.
The survey is also where conflicts are identified, such as utility routing, tight access or site features that force a different layout.