Larry,
The wattage rating on the attenuator is the power handling capability of the device. Say you have a 30dB attenuator? and apply 100W, 100milliwatts comes out the other end and the rest is dissipated as heat in the attenuator. The 100 milliwatts is +20dBm and is still far too much power to apply to any test equipment. You would then cascade another attenuator of as much as 30dB to get to -10dBm which is a safe level to apply to most test equipment, but the power rating of the second attenuator could be minuscule, since it only sees 100milliwatts. Make sure to connect the high power attenuator to the transmitter under test first. Now another situation is say you have a 10dB attenuator and want to apply 100W to it, 90W is dissipated in the attenuator and 10W comes out the other end. You still need a 100W or larger rated attenuator.
Gary
W9TD
Gary W9TD