Does anybody know what exactly the option to select a speaker format in the export dialog does?

When I select Stereo with a 5.1 mix, the surrounds are definitely included in the export, but they're substantially more quiet than I'd expect. And there doesn't seem to be any way to see under the hood what's happening.

edit: the surrounds are at around -15dB, while it's usually -3dB or -6dB for fold downs. The same btw when done in the batch converter. Really curious what's going on here.

What I'm trying to achieve is to export a stereo fold down of a 5.1 mix and I can't really find a way in S1 and thought this might be the (a) way, but without knowing what it's doing that's not really a great option.

In Nuendo I can just set up an additional stereo bus, setup a send to that bus and it's automatically downmixed while I have the option to decide how exactly it is downmixed. Very convenient and usually S1 is good at convenience But here I can't see how to do this. What am I missing?