The background noise is the signal noise in the absence of external light, including read noise as well as shot noise related to the dark current and thermal background current. It is one of the tests for the detector pixel quality, determined from radiance background measurements at the longest exposure time, corrected for any averaging. The higher background noise since orbit 9388 is due to a longer exposure time.
The nominal version is corrected for any signal drift and is input for the pixel quality that is used by the L01b processor. A plain version is not drift corrected, to see whether there is a significant drift. A biweight version is also not drift corrected, but is less sensitive to deviations from a Gaussian noise distribution, for example when the dark current jumps temporarily (random telegraph signal, RTS).