HP 8712C User Manual page 257

Calibrating for Increased Measurement
Accuracy
Choose an Appropriate Calibration Method
Note that after you have calibrated, a "C" appears in the upper right hand
corner of the display. This "C" indicates that a user-deEned cal (not the
default) is in use. If you change to a narrower span, note that the "C"
changes to "C?", indicating the analyzer is now interpolating between
calibrated measurement points. The "C?" notation also appears when other
system parameters, such as power, number of points, or sweep time, have
changed.
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