When a Fighter (or any other class, for that matter) has additional iterative attacks because of high bab, he must make the Full-Attack action to get them by default. Other actions have their own limitations. For example, if you used Greater Cleave, you can cleave to as many targets as you want so long as you keep scoring hits. Even if you had only 3 iterative attacks, you could potentially attack all 9 targets adjacent to you using Greater Cleave, Whirlwind Attack, or other similar abilities. Conversely, if you use the Attack action (not Full-Attack) or any other Use Feat or Use Special Ability that defines its own attack cap independent of Bab iteratives, you still obey that cap even if your number of iterative attacks is higher.
