Arudha Lagna (AL) is the reflection of the 1st house—the image the native projects to the world. A person with Ascendant in Libra (diplomatic, artistic) but AL in Aries (bold, pioneering) will be perceived as more assertive than their chart's natural tendency suggests.
Strong AL ruler (well-placed, in own sign, exalted, in friendly sign) = public respect, career success, visibility. Weak AL ruler (debilitated, combust, in enemy sign) = perception is undermined; native may be capable but is not recognized.
Upapada Lagna (UL) governs marriage and partnerships. UL sign, its lord's placement, and the state of Darakaraka relative to UL all predict marriage timing and quality. If DK is in the sign of UL's lord, marriage is strongly indicated.
Key principle: Darakaraka ASPECTED (via rashi drishti) by AL ruler = marriage is likely. DK in AL = spouse is a public figure or marriage brings public recognition. DK in 12th from AL = marriage is secret or delayed.
Upapada Nakshatra: The nakshatra of the Upapada Lagna cusp is said to match the native's spouse's nakshatra (classical text: if native is Ashwini, spouse is often Ashwini too). This is empirical; test it on your own charts.
The engine computes AL and UL, their rulers, and interactions with DK. You will synthesize these to predict marriage timing and spousal compatibility.