HP 8712ET User Manual page 263

Transmission
Tracking Transmission tracking is a transmission measurement
error term. It is a measure of the corrected
through-standard.
A typical transmission tracking error is ±0.01 dB. Most
likely this term will be dominated by trace noise.
Typical Transmission Tracking Error Term
Figure 6-9
ET User's Guide
Calibrating for Increased Measurement Accuracy
Checking the Calibration
6-37