In 1984 NATO adopted a doctrine of battle known of as ‘Air Land Battle’. This doctrine divided the battle space up into a three parts - the Rear Area, the Main Battle Area and the Deep Operations. Generally speaking, different elements in the force were given responsibility for separate areas with clear boundaries on those areas. The areas given to specific units generally fit the capabilities and limitations of the forces assigned to those areas. For example a B-52 would have responsibility for targets in the Deep Operations area because of its ability to deliver large amounts of highly explosive ordnance from the relative safety of high altitude but its inability to actually hold any ground. An Infantry Company or Tank Platoon would be given responsibility in the Main Battle Area due to their relatively short range, compared to the aforementioned B-52, and their ability to take and hold ground, which the B-52 lacks. Artillery with its longer range and high damage potential but relatively low survivability, compared to the Infantry and Tanks, is more suited to the Rear Area where it can support the Main Battle Area with relative safety.
Now, I know that right now you are doing one of two things. You are either:
A) Wondering who in the hell cares about all this mumbo jumbo.
B) Already categorizing units in your army based on the above criteria.