Question How is official cause of death determined when someone had more than one illness?

May 3, 2020
Preferably answers would come from people who actually work with death certificates either filling them out or for statistical/data research projects. Sorry if this isn't the best forum, couldn't find one that seemed a better match; please move if necessary.

Are there official rules/guidelines for what type of cause is supposed to be documented on a death certificate? If yes, how does this actually work in real life, as opposed to how it is "supposed" to work?

I ask because this seems to have a dramatic impact on how to view disease fatality impact.

1."On re-evaluation by the National Institute of Health, only 12 per cent of death certificates have shown a direct causality from coronavirus, while 88 per cent of patients who have died have at least one pre-morbidity – many had two or three," he says. - about deaths in Italy.

2. reading about the Spanish Flu, a paper specifically mentioned that the numbers they chose to use included both flu cases and pneumonia deaths as the two had such a high overlap (sorry couldn’t find the link right now)