It is not in the Core rules, it is a SSR (special scenario rules). Here is the relevant text:
Starting turn 3, the U.S. Player may call in 4 air strikes with a maximum of one air strike per turn.
Procedure: Only Cpt. Peters may call in airstrikes. In his impulse, he picks a hex in his LOS that
is currently occupied and spotted or was occupied and spotted on the previous turn. He places a
spotting round marker and rolls two dice. The white dice is viewed. On a 1 or 2, it is a direct
hit, on a 3 or 4 it lands one hex in the direction of the colored dice. On a 5, it lands two hexes in
the direction of the colored dice. On a 6, it lands three hexes in the direction of the colored dice.
The airstrike lands immediately. Flip the spotting round marker to the fire for effect side.
Two airstikes are 500 lb. Bombs which attack the target hex and all adjacent hexes with 5 FP.
Two airstrikes are Napalm which attack the target hex and all adjacent hexes with 4 FP and burns
for 3 turns. Units cannot stay in a napalm hex and must move out of it in their next impulse.
Units moving through or staying within any napalmed hex suffers the attack as it moves through
each hex or at the end of the turn if it sat in the hex without movement. In the admin phase,
replace the fire for effect marker with a smoke 1 marker and the next turn’ attack becomes 3 FP.
In the next admin phase, replace the smoke 1 marker with a smoke 2 marker and the final turns
attack is 2 FP. Each hex in the pattern is considered smoke for all three turns.
