This lists the tracks and busses that are present in the session. The list order reflects the order in the editor, and track or bus names can be dragged-and-dropped in the editor list to re-order them in the editor. The columns in the list represent the following:

V	Whether the track or bus is visible; they can be hidden, in which case they will still play, but just not be visible in the editor; this can be useful for keeping the display uncluttered.
C	Whether the track is visiable on the Cue page, in which case it can be used for non-linear sequencing in the Cue window, but any content in the track visible in the Editor window will not be played.
A	Whether the track or bus is active; inactive tracks will not play, and will not consume any CPU.
I	For MIDI tracks, whether the MIDI input is enabled; this dictates whether MIDI data from the track's input ports will be passed through the track.
R	Whether the track is record-enabled.
RS	Whether the track is record safe; a record safe track cannot be armed for recording, to protect against a mistake.
M	Whether the track is muted.
S	Track solo state.
SI	Track solo-isolated state.
SS	Solo safe state.

Each icon in these columns can be clicked to toggle the track/bus state, which is a very fast way to set multiple tracks/busses state at once.

As with the region list, hovering the mouse pointer over a column heading shows a tool-tip which can be handy to remember what the columns are for.