This is taken from the Editor manual which really gives you an idea...
HQ Type This is only meaningful for HQ units (those units in slots less than 216). This HQ type number
determines what kind of HQ it is and how many hexes away from the HQ it can impact operations. There
are 5 types of HQ types and their numeric values are: Command (100), Corps (0), Air (50), Naval (20)
and Amphibious (30). The type number that should be entered into the HQ Type field is equal to the HQs
value in parenthesis listed above plus the radius in hexes that the unit can impact (this number should be
a number between 1 and 9). So if you are creating an Air HQ and you want it to have a radius of 6, you
should enter 56 (50+6) in the HQ Type field. Command and Corps HQ’s impact ground combat (on
defense the nearest of either type will apply, on attack, both the nearest command and corps HQ can help
out). Air HQs impact air group operations. Naval HQs impact ship repair (only if in the same hex with the
repairing ship) and TF loading speed. Amphibious HQs impact casualties taken by forces making
amphibious invasions (only if in the hex of the invasion).
to expand. You have ground, air and naval HQs... (a fourth is designed souly for landing at hostile beaches, Amphibious HQs).
Most "command" HQs will have a 9 hex radius, which offer bonuses to the units able to be in that radius. Both for offence and defence. Look at the HQ. At the top left, you will see a command radius, as well as what type it is ...
from there, you can plan on moving them as you make your assault, or, better still, know your going to be in an area where combat is going to be awhile, and want more favurable odds for your units. HQs are sometimes over-looked

dont !