What does base proximity exactly mean?

[Cpack]

For example:

# Combat Modifier: Base Proximity 0.15

or

# Combat Modifier: Base Proximity 0.80


Who is related? Airforce, navy or army?

Can anyone please explain the different effects of changing this value?

Thanx

 

[Lord Finnish]

as far as I know it's for interceptor for fighting near home airbase.