How to interpret divisional charts exactly?

Earlier, I saw someone was asking about married life just by independently mentioning placements and combinations in D9 divisional chart. How can using divisional charts independently work for interpretation or prediction? Isn't rasi chart primary and divisional charts should be checked for more or in depth details, upon the rasi chart?

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This is all so strange to me. for example, if we can take D9 independently for marriage, then what is the point of 7th house is rasi chart?

If

7th house,

7th house lord position , dignity and nakshatra in rasi chart,

D9 ,

Upapada lagna

if all of these can be taken independently, there is a high probability where atleast 1 of them shows some negative combinations for marriage. Can that accurately predict a troubled marriage?

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So how exactly should one interpret divisional charts? Can they be used independently? If not, how to read them along with rasi chart?