I don't know if this is any help, and I don't know if all devices are the same, but the MIDI manual for my Yamaha MU90r says that M is the channel (singular channel, not channels)
that is set to Mode4, and notes on that channel ONLY are received monophonically..

But the data range is given as 0 to 16. I'm wondering if 0 is therefore in effect All Channels, i.e. Omni OFF, while 1 to 16 means Omni On, i.e. that channel ONLY

Geoff

The Complete Specification PDF has a detailed explanation, in pages 20-24.

As Pedro mentioned, in the MIDI 1.0 Detailed Specification PDF, MIDI modes are described on PDF pages 25 to 29 (printed page numbers 20 to 24). However, I know I have found the description of MIDI modes confusing. Here is a description of MIDI modes that helped me understand them a little better: MIDI channels, voices, timbres and Modes

The main example use of Omni Off Mono mode (Mode 4) with multiple channels is a MIDI guitar controller that is set to send each string on a different channel. It will send mode messages to set the receiver to the Omni Off Mono 6-channel mode.

If the receiver's Basic Channel is set to Channel 1, the sender should send:

B0 7C 00 [Control Change Channel 1, Controller 124, Value 0 (Omni Off)]
B0 7E 06 [Control Change Channel 1, Controller 126, Value 6 (Mono On, 6 channels)]

Then the individual strings will be sent on channels 1 to 6.

If the receiver's Basic Channel is set to Channel 2, the sender should send:

B1 7C 00 [Control Change Channel 2, Controller 124, Value 0 (Omni Off)]
B1 7E 06 [Control Change Channel 2, Controller 126, Value 6 (Mono On, 6 channels)]

Then the individual strings will be sent on channels 2 to 7.

Each individual string's channel is monophonic (maximum of one pitch at a time), but when you consider all six strings' channels together you can hear up to 6 pitches at a time.

The advantage of using 6 monophonic channels is that they can have independent Pitch Bend. This lets you represent bending one string on a guitar without affecting the pitch of the other strings. (If you use 1 polyphonic channel, Pitch Bend affects all the notes on the channel at the same time.)

MIDI MODES ARE OLD

Be aware that MIDI modes seem to be designed for a time when MIDI receivers were only able to produce one instrument sound at a time (mono-timbral) and were focused on receiving one channel of MIDI data. A typical receiver was mainly intended to be set to listen to one MIDI channel (its Basic Channel) and ignore the others. To get multiple instrument sounds, you were expected to connect multiple receivers and set each to listen to a unique channel.

Modern MIDI receivers can produce more than one instrument sound at a time and listen to all channels as unique channels. This is called "16-channel multi-timbral". A single physical receiver that is 16-channel multi-timbral can be considered to contain 16 virtual single-channel receivers. Each of the virtual receivers is set to listen to one of the channels from 1 to 16, and is always in Omni Off and can't be changed to Omni On.

Sometimes modern multi-timbral receivers don't implement Mono modes at all. In that case, each of the 16 virtual receivers can be considered to always be in Mode 3 (Omni Off Poly), and the mode messages only have the same effect as an All Notes Off message for the channel.

This means when using modern 16-channel multi-timbral receivers, the mode messages are usually not important. Understanding the mode messages is mainly an exercise for the technically curious, or those that happen to have a MIDI receiver that actually uses the mode messages.