This option merely selects between "average" sky background conditions, and "high" and "low conditions.
Values assumed are:
Low = 23.3 V mag arcsec-2 which represents minimum zodiacal light for sources at high ecliptic latitudes.
Average = 22.5 V mag arcsec-2 which represents average zodiacal light.
High = 21.0 V mag arcsec-2 which represents maximum zodiacal light + reflected earth light.John Biretta <email@example.com>, STScI