Net Price (roll-up) and roll-up flag not correctly working

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Hello,

We are trying to use the Net Price (roll-up) and roll-up flag standard fields but they seems not to work correctly.
There is not a clear documentation about that but in our quotation we added two different products with only different configuration options and we have for the first one the two field populated, while for the second one no.

The working of this functionaliy seems to us randomic so we can't say based on what the Net price (roll-up) and roll-up flag are working.

Can somone provide us the formula applied to the two fields as there is not a documentation about that?