The phrase sect light is used in astrology to refer to the luminary which is of the sect in favor in a particular chart, which would be the Sun in a day chart, or the Moon in a night chart.
Since the Sun is the “light” or “luminary” that is associated with the day or diurnal sect, it is the “sect light” by day. Conversely, since the Moon is the “light” or “luminary” that is associated with the night or nocturnal sect, it is the “sect light” by night.
Essentially the term is just used as a quick way to reference the luminary that is of the sect in favor in the particular chart that is being examined, as opposed to the luminary that is contrary to the sect in favor in the same chart.
The term was often used by Greco-Roman astrologers during the Hellenistic tradition. In some texts the sect light is referred to as the “light of sect”.