Sonos Architectural 5.1 or 7.1

@Mission 

Very interesting… never took a look at that document before. But I just can imagine that the document describes the positioning of the speakers in general. If you use them out of a Sonos system, they could be connected for example to any Dolby Atmos or 7.1 Amp as front, center, highs (just for Atmos), side rear or back rear speakers. Not sure, why this is mentioned in a user manual of a Sonos optimized product. 
In a Sonos home theater setup as rears as far as I know you can only use two of them connected to one Sonos amp. 
And that imo with a Sonos Arc and Sub will be a 5.1.2 Setup. You won’t get separated back rear channels with such an setup. Only Era300 or Era100 as rears support that. 
In the end as rears in a Sonos ht setup I recommend placing the Sonance in ceiling speakers mainly at the sides (as shown in the 5.1 documentation) but a little bit (about 30cm) behind listening position. 
That‘s the way I did with my Sonos OneSLs and even with Atmos audio it sounds great. Non Atmos supporting Sonos speakers as rears in a Sonos Atmos setup combine the back rear and side rear channels but mainly are used for the side rears. 
 

Thanks for the replies. Ya I dont think it makes sense. There is only 1 set of speaker connections on the Sonos AMP. How would it know how to separate the speakers...

I guess the next follow up question for this is does anyone see a benefit in running 4 in ceiling speakers running off of the 1 amp even though it’s only 2 channel? I would actually lean towards the placement set out in that first manual with 2 spread out at the sides and the other two more centered but further back.

Just wondering if there would be any benefit to this with the additional cost of extra in ceiling speakers...Would be running the ARC at the front of course along with  a Sub

@Mission

Not sure if the Sonos Amp can be used with 4 speakers in surround mode at all. I suppose it can, but found nothing official about that in the support docs.

But even if that would work, I don’t recommend to do so. Rear signals (if they aren’t separated ones like with Atmos) have to came from ONE position each side. So 4 speakers imo won’t be a benefit and not worth the money. I would try to find the best position for your ears with two rearspeakers. There’s no strict rule where to place them, just recommendations. A compromise of somewhat to the sides and a bit behind your listening position should be ok.

Be aware that in ceiling speakers generally aren’t the best choice for rears because the best placement would be on ear level. But also that point can differ as to be a design compromise.