For example, the Keysight 3458A 90-day DC Voltage accuracy specification for a 1 V nominal reading in 1 V range would be 4.9 uV. When using this accuracy specification in an uncertainty budget one needs to know what confidence level is assosiate with the specification e.g. 95,45% (k = 2) or 99% (k = 2.576)?
HPAK usually uses 95% / k=2, confidence level.
In the Keysight metrology videos (#1-7), about GuardBanding, a hint can be found.
In a more detailed 3458A calibration certificate from KS Malaysia, I found a dedicated statement, that this instrument was calibrated at 95% / k=2.