Hellenistic astrology has three major subdivisions. These are universal astrology, natal or genethliological astrology (which means “pertaining to the birthday”), and katarchic astrology (which means “pertaining to a beginning”).
Natal Astrology studies the events occurring in an individual human life from the chart cast for the moment of his or her birth.
Universal astrology is what modern astrologers would call "mundane astrology," dealing with the prediction of the weather, geophysical phenomena such as earthquakes, and events affecting entire countries, cities, or masses of people, such as plagues and wars. Following ancient practice, earthquakes and thunder were also used as omens in their own right, independently of astrology.
Katarchic astrology includes both electional and event astrology. Electional astrology seeks the best time for performing some action by studying the charts of the different proposed times for commencing the action. Event astrology tries to understand everything about an event from the time at which it took place. For example, suppose you receive a letter or message from someone and you wish to know the intentions of the sender, whether the message was tampered with in some way, etc.; to find these things out, cast the chart for the moment you received the letter. Or suppose you wish to know the course of someone's illness; cast the chart for the moment he took to his bed.
To date, there is no good evidence that Horary astrology was practiced during the Hellenistic era. This kind of astrology attempts to answer a given question for a client by constructing the chart for the moment that the question was asked.