Which formats are supported for multichannel speaker calibration?

What exactly do we mean with 'supported formats'?

In this case, supported formats is referring to discrete channel layouts for multichannel setups. Meaning, how many physical channels there are, and what is their configuration. 

This shouldn't be mistaken with ATMOS solutions, where the number of objects can exceed the channel count (having more speakers than physical channels via a digital routing solution).

Which formats are supported?

SoundID Reference supports calibration for up to 16 individual channels in a variety of multichannel formats, starting with 2.1, going all the way up to 9.1.6 (Surround, ATMOS, Sony 360, etc.).  Here is the full list of supported formats:

• 2.0 Stereo (subwoofer can also be used, but only in a stereo configuration: L and R channels routed through the sub, using the subwoofer's internal crossover)
• 2.1 (subwoofer on a separate channel)
• 2.1.2
• 3.0
• 3.1
• 3.1.2
• 4.0
• 4.1
• 4.1.2
• 4.1.4
• 5.0
• 5.1
• 5.1.2
• 5.1.4
• 7.1
• 7.1.2
• 7.1.4
• 7.1.6 Overhead
• 7.1.6 Overhead/Mounted
• 9.1.2
• 9.1.4
• 9.1.6 Overhead
• 9.1.6 Overhead/Mounted

What if I have two (or multiple) subwoofers - isn't that an x.2.x format? 

Multiple subwoofer setups are also supported, however, they would still be routed through a single discrete channel. So even if you have multiple subwoofers in your multichannel setup, that is still technically an x.1.x setup.

In other words, the second decimal refers to a discrete channel count for the subwoofer setup, not for the actual subwoofer unit count.