Keysight Technologies 8990B User Manual page 81

Using the 8990B 2
Thresholds
Set the measurement threshold value in percentage.
Trace Ref Lv 1 and 2 (%)
Use in the calculation of transition durations and occurrences.
Pulse Duration Ref Lvl (%)
Allow pulse duration measurements between non-standard reference levels.
Keysight 8990B User's Guide 81