HP 8712ET User Manual page 259

Source Match Source match is an error term which can be used to
determine the magnitude of re-reflections from the
source port (RF OUT).
Reflection calibrations always correct for source match.
The two transmission calibration types that will
correct for source match are Enhanced Response and
Test Set Cal. (Test Set Cal is only available when using
your analyzer with a multiport test set. If you are using
a multiport test set, refer to its User's and Service
Guide for more information.) A corrected source match
of < −53 dB indicates a good calibration for these two
calibration choices. See
Match (Corrected) Error Term."
ET User's Guide
Calibrating for Increased Measurement Accuracy
Checking the Calibration
Figure 6-6, "Typical Source
6-33