Clearance from any combustible materials like siding, decks, plants and trees.
Clearance from any intake air like doors, windows and vents.
Clearance from any equipment that may need servicing like a electrical and gas meters.
Any restrictions that may cause the unit to overheat.
Any conditions that may cause it to have flooding or excess snow cover.
Visual aesthetics and least amount of upkeep.
Noise reduction and lifestyle.
These are the basics of unit placement but you should have a site visit to verify all of these specs as well as any local codes that may apply. All jurisdictions have different codes and are not always the same.