HP 8712ET User Manual page 265

Reflection
Tracking Reflection tracking is a reflection measurement error
term. It is a measure of how well the open or load
standards have been corrected.
A typical reflection error tracking error is ±0.02 dB.
Figure 6-11 Typical Reflection Tracking Error Term
ET User's Guide
Calibrating for Increased Measurement Accuracy
Checking the Calibration
6-39