Speaker arrangement terminology

The first number is on the floor plane.

The second is the number of subs

The third number is overhead.

So a

5.2.4 would be 

5 floor speakers

2 subs and

4 overhead speakers

again 

5.4.4

5 on the floor

4 subs and

4 overhead..

so a 

20.10.20 would be a bunch of work.. ;-)

"...The second is the number of subs..."

It’s the number of discrete LFE channels, see below.

I had to think about this (ouch). I think the point one, point two etc., is used in home theater systems with a LFE outputs. A limited frequency band output from the AVR. One point for each channel of LFE. On the other hand in two channel systems with a pair of subwoofers they are really just a separate driver and another "way" of a loudspeaker system. A way being a frequency band of operation. My woofer towers for example can operate outside the band provided by a LFE output. So that system is 2.0 even though the woofers are a separate enclosure. If that system ran pure subs from LFE outputs, it would be 2.2. A least that is the way I see it.

An additional thought; The number of subwoofers does not keep adding additional numbers, it would be the number of LFE channels. One, two, or ten subwoofers run off the same channel is still just point one, same as a single sub with multiple drivers.

Yes, the number after the dot is for subs.  Height speakers like in Dolby Atmos count before the decimal.

However, having said that, this is usually talked about by vendors of equipment.  Like 5.1 or 7.2 receivers. It doesn't make a lot of sense/need for the user.

If you have two subwoofers, but use a Y-splitter cable on one output of your receiver/processor or if you daisy chain one sub to the other, then it is still considered 1 LFE channel since you are only using one output on your receiver/processor.  So, it would be 6.1 if you were using 6 floor channels.

It would only be considered 6.2 if you used two separate outputs on your receiver/processor to drive two separate subwoofers as two individual discrete analog channels.  In this case, the receiver/processor may send certain sounds only to one of the subwoofers, such as bass from the left main speaker/channel if you have that speaker set as "SMALL".  Any LFE effects from the movie (such as earthquakes, explosions) would be sent to both subwoofers.

An output of the Ayre preamp goes to a Velodyne SMS-1 bass manager that provides acoustic room correction for a pair of HGS-15 subs. Each sub is connected via a separate balanced line, but the bass is monaural, so by russ69’s counting my setup is 6.0, with the subs being extensions of the front LR.

For surround, HDMI goes to a Bryston SP3 that returns balanced front LR to the Ayre preamp and thus uses the same subs, so still 6.0. The SP3 does have an unused LFE output, and I have a pair of spare HGS-10s it could feed. I suppose that would then be a 6.1 setup.

My plan is to use a pass through from the SMS-1 to a second SMS-1 with a long balanced line to a third HGS-15 at the other end of the room.