In brief: The Sun above the horizon gives a day chart; below the horizon gives a night chart under the basic sect rule.
An astronomical starting point
Sect begins with the Sun’s local horizon position at the chart’s time and place. If the Sun is above the geometric horizon, the chart is diurnal; if below, nocturnal. The Ascendant–Descendant line represents the horizon in a standard chart drawing. At or very near sunrise or sunset, a small time, coordinate, atmospheric, or horizon-definition change can alter the classification. Report the actual Sun altitude and the rule used when a result is close.
The traditional groupings
A classical scheme groups Sun, Jupiter, and Saturn with the day sect and Moon, Venus, and Mars with the night sect. Mercury is handled more flexibly and different authors give it additional criteria. These assignments are symbolic historical categories. They do not mean that Saturn physically behaves differently at a person’s birth after sunset, nor do they establish a causal effect on the person.
How a reader uses sect
Some traditional techniques adjust the interpretation of a planet according to whether it belongs to the chart’s sect. The same chart might also use day/night variants for the Lots of Fortune and Spirit. The latter are explicitly mathematical formulas; reversing the day/night choice reverses which luminary is added or subtracted. A responsible display makes the classification and formula visible instead of silently choosing one branch.
What an uncertain time changes
A daytime or nighttime birth far from the horizon is usually easy to classify with a reliable civil date, place, and offset. Near dawn or dusk, a rounded birth time or uncertain historical time zone may make both classes plausible. When the birth time is absent, do not assign sect from a noon substitute. Check a time window and, if it crosses the horizon, show both traditional readings as alternatives.