Based on the below link - plus points and MUA's are given the same upgrade request priority. Which actually clears first? Is it based on a combo of status tier and fare class or something else?

I'm looking at trying an MUA for the first time (FCO-IAD) and as a silver (with minimal chance of clearing), i think I would have a better shot by buying PE to upgrade to J rather than trying to go from E to J, correct? I'm not considering MUA from E to PE as I don't want to risk a PE middle seat.

https://www.united.com/en/us/fly/mileageplus/upgrades.html