With your setup, the Arc will probably just play standard Dolby Digital 5.1. The Arc doesn’t do anything to enhance the sound.

If you feed the Arc with Any Dolby codec, the Arc accept the signal and just cuts off the channels it doesn't have.

for example if you feed the Arc Dolby digital 5.1, the Arc plays 3.0 sound (left, right and center). If you feed it Atmos, it plays 3.0.2 (left, right, center, left up, right up) 

The arc won't down sample or up sample channels, it just plays what it gets, so you will never get like 5.0.2 if you don't add rear channels 

Not exactly. Without surround speakers, the Arc alone will still play 5.0.2 with virtual surround using the side speakers.

Thank you both.  And with surround speakers the Arc itself plays 3 channels and but utilises the 2x Ones for surround and the sub as per the Beam?

From a former Sonos employee:

“The Arc on its own is considered to simulate 5.0.2 audio, when you add a Sub, it hits 5.1.2. And if you add surround speakers, it’s still considered 5.1.2. The side firing speakers that were being used for the rear simulation, will adjust to help out the rest of the audio instead, not add additional channels.”

I would also add that with the Sub connected, the drivers producing the bass for the Arc will now be focusing more on the mid range.