Every damaged facility, every site with debris, and every major emergency protective measure taken to protect the public, listed by location with a rough cost. You also say whether you will claim management costs (Category Z) or building code and floodplain work (Category I). It does not need detailed damage descriptions or final costs, and FEMA does not use it to decide how much you receive.
Your damage inventory with the eight required items for each site, plus photos and notes you gathered while identifying damage. FEMA does not use the rough costs to set your funding, so a reasonable estimate is enough at this stage.
Anything left off the impact list is very hard to add later. List every site you suspect, including ancillary items like fences, signs and parking lots, and withdraw the ones that turn out ineligible rather than missing one.