User`s guide

What is an Attenuator? Getting Started with Attenuator Modules
Agilent 81560A, 81561A, 81566A, & 81567A Optical Attenuator Modules, First Edition 21
Calibration is even easier and more convenient if the reference
powermeter and the attenuator are hosted by the same mainframe: All
power related offsets can be determined by a firmware function that
reads a value from the reference powermeter. The difference between
the power value read by the reference powermeter and the actual
value of the attenuator is automatically stored as the offset.