What is the CTRL button supposed to do on the Mackie Control (ie, Ableton Live) mapping?
I found the answer in the Live 7 manual, apparently the last version with Mackie Control usage docs. (?)
Relevant sections are below. I tried them out.
Jog Wheel In the Session View, the jog wheel scrolls through the scenes. In the Arrangement View, the jog wheel moves the play position. To change global quantization, hold down the Mackie Control’s Control button while turning the jog wheel.
Solo Activates/deactivates soloing for the track in exclusion of all other tracks. To solo the track nonexclusively (i.e., in addition to other tracks), hold down the Mackie Control’s Control button while pressing this. Note that a Misc Preference setting in Live allows deactivating exclusive track soloing. When this setting is deactivated, the behavior of this knob with respect to exclusive and nonexclusive soloing is reversed.
Arm By default, this arms the track for recording in exclusion of all other tracks. To arm the track nonexclusively (i.e., in addition to other tracks), hold down the Mackie Control’s Control button while pressing this. Note that a Misc Preference setting in Live allows deactivating exclusive track arming. When this setting is deactivated, the behavior of this knob with respect to exclusive and nonexclusive arming is reversed.