From: Sandviken, Sweden
if i recall correctly:
for air combat TF it is the amount of hexes the TF reacts towards another enemy Air combat TF
for surface combat TF it is the reaction range for surface combat in another hex.
lets say you have a surface combat TF in Manila, and a base to hexes away gets attacked by sea (or a surface combat TF appears); if you set reaction TF on 2, there is a chance your TF will move there if it has enough speed and Ops-points.
Bougainville, November 1,2,3,4,5,6,7,8,9. It rained today.
Letter from a U.S. Marine,November 1943