Applying the theory of sect would allow the ancient astrologer to determine with a higher degree of accuracy which planets act as positive influences in a chart, and conversely which planets are particularly problematic or destructive.
Sect is not a matter of strength. It corresponds more to the flow of energy in a chart. Being "in sect" or "out of sect" will enhance or impede the natural flow of placements without other factors being taken into consideration.
With other mitigating factors aside, a planet contrary to its proper sect acts as a malefic: challenges, difficulties and hardships, both in its natal placement and when it is activated as a time lord or within the context of its transits. But a planet in its preferred sect is a benefic, which sources the person's greatest windfalls, strengths, and areas of relative ease.
Diurnal and Nocturnal Planets:
There are 2 fundamental schools of the planets: those that belong to the daylight and those that belong to the darkness of night. In other words, a chart holder is either of the diurnal sect or of the nocturnal sect (cf. Genesis 1:16). If a day time birth, then the Sun is dominant in the chart, along with Saturn and Jupiter; but if by night then the Moon becomes the predominator along with Venus and Mars. If for example, you are born at night then look to the Moon's position and to the positions of its co-sectarians which are Mars and Venus; but if by day then consider the Sun and Jupiter and Saturn. The roots of this doctrine date back into antiquity but have a firm foundation in Aristotelean natural philosophy.
Jupiter, the Sun, and Saturn are diurnal. This means that they prefer to be above the horizon during the day and below the horizon at night. So in the day chart pictured on the left, these planets are at the top: above the horizon line. On the night chart to the right they are below. Here they are most beneficial. Conversely, the nocturnal Venus, the Moon, and Mars, prefer to be below the horizon during the day and above the horizon at night. Thus in the left chart they are at the bottom, and on the right they rise to the top. Those are their preferred positions in a chart.
An ancient astrologer would also pay attention to which signs these planets are actually in. Nocturnal planets prefer to be in feminine signs, while the diurnal planets prefer to be in masculine signs.
According to Aristotle there were 4 elements; fire, air, earth and water. There were also 4 qualities; hot, cold, wet and dry. The Sun, since it is of the daytime is considered hot and dry and is of the fire element. The Moon is of the night and is naturally considered cold and wet. The night preserves water but the heat dries it up; it tends to be cooler at night. The other planets partake of these elemental qualities also, and Mercury you will note belongs to neither sect because Mercury is common to both. Saturn is naturally cold and dry; this physiological humour is called melancholic and corresponds to fall (autumn). Saturn, Venus and the Moon are naturally cold, Mars, Jupiter and Sun are naturally hot; so why is it that Saturn is of the daytime sect and Mars naturally fits with the nocturnal sect? This is because Sun, Moon, Jupiter and Venus are natural benefics, but Mars and Saturn are malefics and must be counteracted to be in sect. For example, if it is day, Mars is out of sect because the heat of the day becomes excessive. At night, Mars is in Sect, because the heat and dryness of Mars is held in restraint by the cool and moist of night, taming Mars malefic tendency.
To summarize, Mars and Saturn would be best placed in sects opposite to their nature, to balance their natural tendencies and mitigate their effects.
There are two further ways that a planet can be in sect, other than it just being a diurnal or nocturnal birth chart. A planet can be in its own "light" or it can be in its own "similitude." It is not possible to define being in one's own light without defining the sect. Considering a diurnal chart, Saturn and Jupiter are in their own light if they are above the horizon with the Sun, and the Moon, Venus and Mars are also as long as they are below the horizon. In a nocturnal chart, the Moon, Venus and Mars should be above the horizon and Jupiter and Saturn should be below in order to say they are in their own light. In the case of being in its own similitude, the planet needs to be in the sign of its own nature; the Sun should be in masculine signs and Moon should be in feminine signs. The planets of diurnal sect should be in masculine signs and the planets of nocturnal sect should be in femine signs to be in similitude. The exception to this is Mars. According to Arabic astrology, Mars is best placed at night near the midheaven in Aries or some other masculine sign (possibly because Mars is the lesser malefic and does not need to be counteracted as much as Saturn). It should be said that this perception may be due to a misreading of the Greek material at the time.