The Session Record Button (the light red /orange one on top of the Session Window) serves 2 purposes:
1) Making it possible to record more than 1 clip at once.
2) Making it possible to overdub 1 clip, or overdub more clips at once
Lemme explain these 2:
Let's say you have 2 midi tracks in Session View, with no clips in it.
Record arm both midi tracks (holding down CTRL key or CMD key on mac) and pushing both red record arm buttons.
Click the + Button, on top of the Session Window.
This enables MIDI overdub recording.
Click the uppermost clipslot in midi track 1(or the one in midi track 2, or scene 1)
Now push the light red Session Record button.
Now you can see that BOTH uppermost empty clipslots in BOTH midi tracks get recorded.
(appearing RED triangles in these 2 clipslots, meaning that they get recorded)
So hit some midi notes, which get recorded in BOTH clips at once.
Now hit the light red Session Record Button again.
The 2 red triangles in the 2 clips turn green, meaning that they are in play mode now.
Hit the light red Session Record Button again.
The 2 green rectangles turn red again, meaning that midi can be recorded again in these 2 clips and the midi data is ADDED to the existing midi data in the 2 clips.
This overdubbing method, (by pressing the light red Session Record on and off repeatedly) works also for just one midi track that is record armed instead of 2 (or more).
Hope this helps abit ..
Rob van Hees
Macbook Pro 15", 8 Gig Ram, Mavericks 10.9, Novation Impulse, Maschine MKII, Live 9.1 (latest beta 64bit), Logic Pro X 10.04, Protools 11.02, just too many plugins.