For reference to ninth house read the following post:
The D9 chart is also used to determine the spiritual inclination of a native. In the below post I have described how a native can determine his or her Ishta Dev.
As per BhagyaSamhita rules, Atma Karaka is the planet that has the highest longitude (or degrees) in one's natal Chart. Atma Karaka in Navamsha is named the "Karakamsa". The 12th from the Karakamsa controls the "IsthaDevata". The 12th house from the karakamsa is named the "Jivanmuktamsa". This is the house which suggests the freedom of the soul and thus we have to observe this house in order to find your IshtaDevata and if there is a Graha in the Jivanmuktamsa, it indicates the Devata. If there are more Grahas than one, we select the strongest. If there is no Graha in the Jivanmuktamsa, we can see whether any Grahas are inspecting the 12th. If no Grahas aspect it, we can take the 12th Lord as IshtaDevata. In the chapter called "kaarakaamsaphala" in BPHS(Brihat Parasara Hora Shastra), Parasara stated the deities adored based on the planets in the 12th from kaarakaamsha. For example, Sun in 12th from Atma Karaka shows a believer of Shiva and Moon in 12th from Atma Karaka shows a worshipper of MaaGouri.
Here are given the ruling deities for each planet who can be your IshtaDevata as per your Horoscope.
Sun - Vishnu, Rama, Shiva
Moon - Krishna, Shiva, Parvati
Mars - Hanuman, Subramanya, Sri Narasimha
Jupiter- Vishnu, Sri Vamana, Dattatreya, Sai Baba
Venus- Maha Lakshmi, Parashurama, MaaGouri
Saturn- ShaniDev, Hanuman, Kurma, Shiva, Vishnu
Rahu - MaaDurga, Varaha
Ketu - Ganesh, Matasya
Ascendent - Kalki
Besides determining one's Ishta Dev D9 also indicates how much spiritual a person will be. Actually, if you are not a following of ancient Indian philosophy of Sanatan Dharma (now termed as Hinduism), you will not be able to grasp the above stuff. But still you can determine one's propensity, because in principle, the Lord or the almighty is one. It does not matter which path you follow, if your D9 chart is strong, you will have a natural inclination towards the ONE almighty. It is just that Sanatan Dharma has so many deities, they are just replicas of ONE Lord.
Hence, if you have a strong D9 chart (also strong D60 chart) you will be a spiritual person and you will follow a path of goodwill, leading to emancipation.