What does the actuator name for URDF reference?

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For this question I am using the code sample provided at the documentation:

http://www.ros.org/wiki/urdf/XML/Transmission

<transmission type="SimpleTransmission" name="caster_front_left_trans">
  <actuator name="caster_front_left_motor" />
  <joint name="caster_front_left_joint" />
  <mechanicalReduction>1</mechanicalReduction>
</transmission>

I understand for the most part what most of the names refer to, but I am still puzzled as to what exactly the function of the actuator name is. I did my best to grep through the shipped packages, but with no luck. Does the name reference something external to the robot model? What does it reference?

I suppose I should note I was looking at the PR2's URDF description and grepping for its actuator name(s).

Thanks for any help!