Max for Live is a component of Ableton live, which allows MAX (a visual programming language, see www.cycling74.com) patches to be created and operated within Ableton Live. Such patches can be used to manipulate MIDI, audio, and video, thereby massively expanding the range of applications for Ableton Live.
Max for Live can also be used to observe and control almost any element of Ableton Live, by accessing the Live Object Model (LOM) (see below for more details).
This is achieved by using Max for Live objects like live.observer and live.object. Useful starting tutorials on this topic can be found here:
In practical terms, Max for Live allows you to use motion and touch input from the Motion Synth to control Ableton Live in virtually any way you can imagine.
Here is an Ableton Live project file that demonstrates a max4live patch:
The purpose of this patch is to switch control of any particular effect on and off via touch (and then change the level of that effect via orientation or motion). So when the user is touching a specific button they have control of a specific effect, and when they let go of the button they are no longer in control of that effect, and the user is free to move without controlling any effect. Multiple effects could also be controlled by simultaneously touching multiple buttons.
When using this patch, try triggering the clip and send a C2 note (48) to Ableton Live. The max4live patch in the MIDI track should turn the beat repeat effect on and off in the audio track. The beat repeat effect will be on (while the note is held) and off (when the note is released).
You can edit the max4live patch to change the note number and the target object you want to control (by changing the path). While editing, command-click any object to access its help file and get a better understanding of how it works.
When using this max4live patch you may want to turn Octave Control off: http://auug.info/wiki/doku.php/public/notes_tab
If you wanted Octave Control on, the solution would be to enable the max4live patch recognise a note value regardless of octave (e.g. C3, C4, C5, etc). In max4Live, this would be achieved through the use of a modulo operator: https://docs.cycling74.com/max5/refpages/max-ref/modulo.html
Max for Live also has a free library of thousands of Max for Live devices for download:
If you can't find a device that does exactly what you want, you can immediately start editing to achieve the functionality you need.
A very helpful user-created Max for Live device, that can be used to find the 'address' (ID) of the element you wish to observe or control, is called the LOM Navigator. This can be found via the link below, downloaded, and then dropped into any track within Ableton Live: