HP 8712ET User Manual page 262

Calibrating for Increased Measurement Accuracy
Checking the Calibration
Load Match Load match is a transmission measurement error term.
It is a measure of the re-reflections contributed from
the match of the through-standard and the receiver
port (RF IN).
A typical value for this match term is < −22 dB.
Figure 6-8 Typical Load Match Error Term
6-36 ET User's Guide