Phil, All I want to do is raise the volume of one recorded sequencer track in the middle of a SONG (mode). In other words, the volume of this track in the beginning of the song is 85. At measure 90, I want to raise it to 125 and then keep it there to the end of the song. I don't need the increase to be gradual, just need it to happen all at once at the beginning of measure 90. Can I simply insert a channel message on the track that I want to change volume? Could you please explain?

You can INSERT a message (CC7 =125) at the measure you want this Track to change Volume

On the Main SONG screen:
Advance the Song Sequencer to Measure 90
Select the Track in question
Press [EDIT] to view the event list for this Track
Select [F5] INSERT
Set it to “Event Type” = Control Change
Insert your CC7 = 125 at the appropriate Measure:Beat:Clock

CC7 Main CH Volume vs CC11 Expression
CC7 is the Volume of the Mixer Channel. It sets the maximum Volume for the MIDI Channel.
CC11 is musical Expression. It set the instrument’s volume the one you would manipulate during a real-time performance.

The Mixer Volumes allow you to balance one Track with the others… much like the engineer does at the mixer.

If your original Track Volume was 85 that’s CC7 = 85.
If you assign a Foot Controller to do CC11 when the pedal was fully in the toe down position CC11 = 127, that would equal CC7 = 85.

The reason for this, say it was an B3 organ sound, when playing organ your Expression pedal controls your performing volume… it’s the musicians performing volume.

The organ is plugged into the mixer… the sound engineer determines how loud the organ is compared to other instrument… by setting a Channel maximum with the CC7 setting.

Expression works within the ceiling set by the Channel Volume.

Because you change is not a musical change (where you are performing a musical crescendo) but is really a change in the Mix, CC7 is the most efficient method to make the change.
Changing CC11 to maximum will not make it louder than the CC7 (Mixer) setting!

I tried and did not get the desired result. Even though I only added one CC 07 event - 125 at measure 90, my volume increased starting from the beginning (measure 1.) Why? There are no other Control Change events and my track volume was set to 85. What happened / what did I miss do wrong?

Please check my understandings about volumes / outputs and how control messages affect them. You said earlier that my starting Track Volume BEFORE I add any Control Message events is my starting CC 07 level (no messages / event need to be inserted.) The manual talks in terms of part volume levels for a mix using sliders and output levels of a voice. Voice output is independent to any mix levels and therefore is not applicable to this situation. Correct? Mix volume = part volume (i.e. the sliders) which = Control message 07. Correct?