HP 54721A User Reference page 19

Channel Setup Menu
Calibrate
Current Frame ATemp This is the temperature change, of the inside
of the instrument, since the last mainframe calibration was performed. A
positive number indicates how many degrees warmer the mainframe is
now compared to the temperature of the mainframe at the last
mainframe calibration. A best accuracy calibration is valid as long as the
mainframe temperature A is within ±5 ®C of the temperature at which is
was calibrated. If the temperature A is more than ±5 °C of the
temperature at which is was calibrated, you must perform a mainframe
calibration before performing a best accuracy calibration to ensure that
the
best accuracy calibration is valid.
Channel 1 Last Calibration
The oscilloscope displays Best, Normal,
or Uncalibrated, depending on what calibration was last performed on
the plug-in.
Best indicates that a best accuracy calibration was performed on the plug-in,
and that the best accuracy calibration factors are retained in the mainframe's
memory. For the best accuracy to remain valid, the internal temperature of
the mainframe needs to be ±5 °C of when the last best accuracy calibration
was performed, and no more than 24 hours has elapsed.
Normal indicates that the plug-in is not calibrated for best accuracy. Either
the best accuracy calibration factors were cleared from the mainframe's
memory, or a best accuracy calibration has not yet been performed in this
slot. But normal does indicate that the normal plug-in calibration is still valid
for this plug-in, and all you have to do to gain back tiie 1 percent dc accuracy
specifications is to perform a best accuracy calibration.
Uncalibrated means that the normal plug-in calibration is invalid. You must
calibrate the plug-in to normal accuracy before performing a best accuracy
calibration. To calibrate the plug-in, see the Service Guide that is supplied
with the plug-in.