Mike A Schwab wrote:

Those are the metric equivalent of the gap
inside the chains. The outside width varies
by 6, 7, 8, 9, 10, 10N, 10ND, 11 gears.

Aha, here it is:

Chains come in 3/32 in (2.4 mm), 1/8 in
(3.2 mm), 5/32 in (4.0 mm), or 3/16 in
(4.8 mm) roller widths, the internal width
between the inner plates. 1/8 in (3.2 mm)
chains are typically used on bikes with
a single rear sprocket: those with coaster
brakes, hub gears, fixed gears such as
track bicycles, or BMX bikes. Chains with
3/32 in (2.4 mm) wide rollers are generally
used on bikes with derailleurs such as
racing, touring, and mountain bikes.
Fixed sprockets and freewheels are also
available in 3/32 in (2.4 mm) widths so
fixed-gear and single-speed bikes can be
set up to use the narrower and lighter 3/32
in (2.4 mm) chains. [1]

[1] https://en.wikipedia.org/wiki/Bicycle_chain#Width

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