Marriage and the spouse, dharma, the inner self, and the strength of any graha: the navamsha is the one varga used for everything after D1. A planet in its own navamsha is strong; the navamsha lagna is read as a second birth chart.
Meaning
D9 divides each sign into nine parts of three degrees twenty minutes. It is the varga of marriage and the spouse, of dharma, and of the deeper self behind the rasi; the classical practice is to read every promise of the rasi twice, once in D1 and once in D9, and to trust the navamsha where the two disagree.