The MIDI Control section selects whether CA2600 plays in mono (single-note) or duophonic modes (two-note polyphony) and contains individual CV outputs, as well as a pitch bend depth control. It's basically the incoming "traffic director" of CA2600.
MIDI In- Illuminates to indicate incoming MIDI note data. Betcha never would've guessed that?
Pitch- Pitch CV. This is normalled to the oscillators (Kybd) as well as one of the filter control inputs.
Gate- This is an "on/off" voltage typically used for engaging engaging envelope generators. The gate out is normalled to both the ADSR and AR envelope generators, but can be useful for other purposes.
Velocity- Outputs a voltage upon keydown that varies from 0-5V, dependent on how hard the key is struck. Amongst other things, this is useful for controlling filter cutoff amount and amplitude envelope intensity.
Mod Wheel- Outputs a constant voltage dependent on mod wheel position.
Bend / Semi Tones slider- Outputs a negative or positive voltage as the pitch wheel is moved from its center position. Its maximum output level (i.e., bend depth) is controlled with the Semi Tones slider. At max position, pitch bend range is one octave, up or down.
As shown by the orange Kybd box, pitch bend is normalled to the oscillator FM Control Kybd CV inputs. This means pitch bend will work without plugging in any cables (provided the Semi Tones control is set above 0). Plugging a patch cable into one of the oscillator Kybd inputs will disable the pitch bend connection, but you can always mult another patch cable from the Bend output to the oscillator Kybd CV input(s) to regain pitch bend.
Note Priority- CA2600 will usually be played as a monophonic (i.e. one-note-at-a-time) synthesizer, but it includes ARP's model 3620 keyboard controller "duophonic" (dual-voice) mode.
Last- This is the default voice mode. If more than one key is pressed, the most recently struck note will always take priority.
Low- If more than key is pressed, the lowest note will always sound. This how the original ARP 2600 works (as well as most vintage monophonic synths).
High- If more than key is pressed, the highest note will always sound.
Duo (Duophonic) / Second Voice CV- Duophonic mode is based on the original ARP 2600's primitive duophonic circuit. The primary pitch output is always the lowest key pressed. The Second Voice CV pitch output is always the highest key pressed. When a new key is pressed, the highest and lowest key voltages are sampled and held by the two outputs. If only one key is pressed, the highest and lowest key voltages will be identical. However, as soon as a second key is pressed, the highest key voltage is sampled and sent to the Second Voice output, and the lowest key voltage is sampled and sent to the Pitch output. In this mode, legato playing often gives undesirable results, as any time two keys overlap, even for a moment, their voltages are split between the two outputs.
Duo mode is also the most desirable note priority mode when performing covers of songs by The Captain and Tennille, Donny and Marie, and Peaches & Herb.
Retrigger/Legato- If Retrigger is selected, CA2600's envelopes will retrigger from the start every time a new key is pressed. If Legato is selected, the envelopes will only retrigger if no other notes are currently held. Legato mode is useful for emulating the sounds of some acoustic instruments - for example, the sound of plucking a note on a guitar, then sliding on the string to different notes without plucking again. Retrigger mode generally feels more responsive to play and is useful for more aggressive passages.
Incidentally, almost all vintage monophonic analog synths are legato-mode only (including the original ARP 2600), as this was a technological limitation.