Last Updated: July 8th, 2021
In the zodiac, each sign is assigned a triplicity (element) and modality (associated with the signs' positions in the seasons, suggesting personality). In Siderea these concepts exist as well. Each zodiac is affiliated with an element, which they find easier to control than others when it comes to magic, and likewise each of them are affiliated with a modality that suggests what combat role they're best suited to.
Elements
There are four core elements: fire, earth, air, and water. There are a couple more niche elements as well, associated with the non-astrological zodiacs: light, dark, and mind.
Fire: Affiliated with Ariens, Leos, and Sagittarians. Associated with fire, heat, energy, and lightning.
Earth: Affiliated with Tauruses, Virgos, and Capricorns. Associated with earth, metal, flora, and fauna.
Air: Affiliated with Geminis, Libras, and Aquarians. Associated with wind, weather, flight, and sound.
Water: Affiliated with Cancers, Scorpios, and Pisceans. Associated with water, ice, liquids, and vapor.
Light: Affiliated with Ophiuchi. Associated with light, healing, purification, and holy magic.
Dark: Affiliated with Cetans. Associated with darkness, life drain, gravity, and esoteric magic.
Mind: Affiliated with Araneans. Associated with psychic abilities, emotion, illusion, and precognition.
Any zodiac can use magic of any element, and not all magic fits cleanly into an element, anyway. A zodiac's element only defines a race's natural strengths, and through training even a Virgo could become an expert in fire magic. One's birth date is thought to factor into this as well; an Aries born in Cancer season would be expected to be better at water magic than an Aries born in Taurus season, etc.
Of note is that the three non-core elements are each associated with one of the creators: light is associated with the High Star (Frenzael), dark with the Great Fade (Asena), and mind with the Vast Glow (Humilia).
Modality
There are three modalities: cardinal, fixed, and mutable. These reflect the expected personalities and magic styles of the zodiacs they're connected to. Cardinal signs are expected to drive action and take initiative, fixed signs are expected to have great willpower and resist change, and mutable signs are expected to be adaptable and crafty. Similarly, these are reflected in the natural strengths of the signs when it comes to using magic:
Cardinal: Affiliated with Ariens, Cancers, Libras, Capricorns, and Ophiuchi. Cardinal signs specialize in offensive magic and enacting their will onto the world around them.
Fixed: Affiliated with Taureans, Leos, Scorpios, Aquarians, and Cetans. Fixed signs specialize in defensive magic and maintaining what already exists.
Mutable: Affiliated with Geminis, Virgos, Sagittarians, Pisceans, and Araneans. Mutable signs specialize in support/miscellaneous magic and manipulating themselves.
As with element, a zodiac's modality only implies natural strengths; it's perfectly easy for an Aries to instead train in healing or a Virgo in beating the shit out of people with their bare hands. In fact, modalities are often at odds with usual stereotypes of the races, such as Ophiuchi being associated with healing but actually being cardinal signs rather than either of the more fitting modalities. Birth date is once again thought to play a role in things, with the modality of the sign a person is born under either strengthening the potential of their modality or giving them a boost elsewhere.
Modalities are also associated with the creators. Cardinal, associated with creation and beginnings, is linked with the High Star. Fixed, associated with that which has always existed, is linked to the Great Fade. Mutable, associated with the transience of one's own nature, is linked with the Vast Glow.
also they got cool symbols. check 'em: https://en.wikipedia.org/wiki/Astrological_sign#The_three_modalities
Cusps
Cusps, being part of two signs, are thought to excel in both areas associated with their signs, though usually not to the extent of a singular zodiac. In other words, a Capriquarius would be expected to have a natural skill in both earth and air magic, as well as both cardinal and fixed magic. However, they wouldn't be expected to be as good at offensive magic as a Capricorn, nor as good at air magic as an Aquarius.