MONO / POLY INPUT A GLOBAL CONTROL.
Mono / on off .
Poly / on off . toggle ^
and 0 to 8 - up down . i.e restrict Mono Poly to No change 0 , 1 lane to 8 lanes aka notes to be played / recorded. Chord Entry. Restricted to 1 , 2 , 3 , 4 etc , lanes.
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From the manuals WITH EDITS , 
Polyphonic vs. Monophonic Recording
Sequential (monophonic) recording can often be better when recording drums or monophonic instruments. The Mono button allows you to toggle between recording a single note at a time or multiple notes simultaneously (chords). This is located in the Instrument Properties panel, found at the top right of the interface in Redux and near the top right while in the Sampler, Plugin and MIDI tabs of Renoise.
Entering Chords with the Computer Keyboard
When entering notes step by step with the computer keyboard, they will by default be placed sequentially. If you want to quickly insert chords without live recording, you can do this by holding down “Left Shift” *WHY WHY WHY CRAZY LOL * …. while entering the notes. This will automatically create new note columns to the right and enter the new notes there. THE MONO POLY BUTTON WOULD FIX THAT Holding shift to play a Chord using computer keyboard.
Note that MIDI keyboards will always record chords when pressing more than one key at a time. <— To quote " (monophonic) recording can often be better when recording drums or “ , then why is there no MONO / POLY option globally ??
^^^ GLOBAL CONTROL FOR MONO POLY INPUT? .
could be button put near the transport bar or lower of the editor for quick access for incoming notes.
PLEASE 
Note:
Setting all instruments to Mono on , solved midi keyboard input “restricted to one lane” , but Shift key is still needed for computer keyboard chord entry. So would be nice to have a global control always accessible while in the editor view or on the transport bar at all times. Entering Chords could be more easy.