I guess you are talking about a single-voice version of legato?
Polyphonic legato exist too, but it’s hard to define exactly how it should work (many different implementations), depending on playing style etc.
So, if kept simple, the “Mono” button could basically become a switch between “Mono” and “Legato”
(because otherwise you might think that legato did in fact work on multiple voices…)?
Legato could also be defined as allowing for a phrase to transpose / continue playing when multiple keys are pressed.
This is being discussed here: http://forum.renoise…-phrase-editor/