Citrix UPM containers....search index roaming for Outlook. Does turning this on provide some extra functionality? I thought the Outlook search would roam if the profile was configured to capture everything, or am I missing something?

It will create a separate VHDX in a separate path and move to AppData\Roaming\Citrix instead of AppData\Roaming\Microsoft if you use Microsoft per user roaming. If you use the Microsoft Roaming it will just roam the data with out the need for the Citrix option. I have found the Microsoft roaming to be more stable than using the Citrix option, but I haven't tested Citrix recently...

So this is specifically for Outlook, not Windows Search? I was of the opinion that these days you should just let the MS stuff handle everything search, to be fair, but was just wondering if they were separate things as well

As far as I am aware, willing to be told differently, Outlook is the only thing that uses per user search in Windows Search. But the Citrix thing also moves the OST file into the separate VHDX file, which I forgot to mention. I am also of the opinion that just letting MS handle everything search is the way to go.

Starting with server 2019 all search is per user. If you're on 2016 then you'd have to use something like this to split it out. It was a big issue with fslogix when 2019 first came out because you had to turn off search roaming. Even then there were issues with the search index getting locked at logoff.