Tektronix CSA8000B - Manual (Page 332)

Measurement Reference Parameters and Methods
B- 70 CSA8000B & TDS8000B User Manual
Use a Waveform Database
This measurement needs to be performed using a statistical (waveform) database.
When one is specified, the instrument acquires or computes the targeted
measurement source, then accumulates it into in the waveform database, and
then takes the measurement on the database data. When you select the RZ or
NRZ signal type, the instrument attempts to automatically allocate one of the
four waveform databases it provides to your measurement source.