In most cases the tweeter will have a higher sensitivity (db per watt at distance X) than the other drivers in the system. To bring tweeter output closer to that of the others drivers a series resistor or a combination of a series and parallel resistor (L pad) will be used. Sensitivity is one of the considerations in the choice of a tweeter although it doesn't have to be a primary one. A difference of 10db between woofer/tweeter sensitivity is not unusual and is easy to level out.