They are marked on the mini-map ether with an Agency symbol for agency controlled points or a red C for Cell controlled ones.
Once the player finds a Cell controlled tactical location they can activate a "Tactical Assault". Once this is done a large number of Cell soldiers will spawn near by (particularly at doors of buildings) and attack the player. The player must eliminate the Cell forces which are marked with a red "C" over their heads, in order to claim the tactical location for the agency. There are 27 tactical locations in Crackdown 2 and once all are under Agency control the player will earn an achievement.
Once a Tactical Location is secured, the Cell icon will change for an Agency icon. The player will then be able to resupply & change weapons, as well as request a vehicle to be air lifted in.
Also, by parking a vehicle on an Agency Tactical Location, the player can then request the vehicle be stored for future use. A vehical can only be stored once, but will be availbleto be delivered for an unlimited amount of times thereafter.