It’s 7.0.4.

You should be able to confirm that answer here. (It explains how some channels are virtualised)…

https://www.sonos.com/en-us/blog/the-ultimate-guide-to-a-sonos-home-theater

Thank you! That’s the sort of documentation I was looking for but couldn’t find.

Would placing these Era 100s 90-100 degrees off axis still work? I was hoping it was 5.0.2 because I really can’t go any further back.

In theory, the Era 100s could still take the two surround channels from the Arc (essentially rendering the Arc a 3.0.2 soundbar), while virtualizing the remaining channels for 7.0.4.

Anyone know if that’s the case?

This image on sonos.com is perhaps an indication of the placement of the Era-100 surrounds - I guess you just have to get as close to that as you can, with your own listening environment…

My thoughts are try the speakers in various positions, until you get what sounds/looks best to you and where you sit …and then go onto Trueplay tune the setup.

Hope that assists.

Frankly, I’d trust my own ears over a marketing image….