You are correct in your observation that a single midi track can contain midi data on different midi channels. The Midi channel parameter is the midi out channel of the track. If set to none, the midi data is sent to the same channel as the midi data on the track. So if using a multi-timbral synth, and a single midi track contains midi data on channels 1 and 2, channel 1 midi data will play the channel 1 instrument and channel 2 data will play the channel 2 instrument.
If you specify a midi channel, the midi channel of the data is remapped to that specified channel. So, looking at the example above, if you set the midi channel parameter to 3, the data will play the channel 3 instrument, even though the data is on channels 1 and 2,
Most single timbral soft synths (like Dimension pro) usually receive in omni mode (all channels) by default, so it doesn't matter what channel the data is one, so you can leave the midi channel set to none (default). There can be no conflict with other midi tracks because each soft synth midi track act like a discrete midi port with 16 channels, because each soft synth midi track's output is set a soft synth instrument track so the data cannot go anywhere except to that instrument track.
With multi timbral soft synths , you must specify the midi channel parameter in the midi track to aim the midi data at the right instrument in the multi timbral synth.
Let me know if this makes sense.