It allows you to specify, at any time, which of your MIDI controllers that should be allowed to record notes.
By default, Renoise always records MIDI when edit mode is enabled (the record button + red border around pattern). This tool offers a lot more fine-grained control by allowing each controller to be record-enabled independently of one another, either manually or by listening to specific criteria (is the assigned track silent? Is it muted?).
To use it, launch the tool and it will present a list of all instruments that specify a MIDI input:
Click the record-arm button (or use the provided MIDI mappings) to toggle manual record-arming for a given instrument
(alternatively, you can synchronize record-arm with edit mode in Renoise by enabling this feature in Options).
A bright red button means that it's enabled and ready to record. A dark button means that it's not.
A dimmed red button means that it's record-armed, but some condition has temporarily disabled recording - for example, that the assigned track is muted or silent.
You can add any number of instruments and control their MIDI settings from the MidiPerformer GUI. These settings are exactly the same as in the Renoise counterpart (Instrument panel > MIDI)
HOW IT WORKS (LIMITATIONS)
The tool is jumping through a few hoops in order to achieve it's goal. First of all, you need to let it wrap the assigned tracks in a group track. This is the trick that makes it possible to "block" notes (otherwise, they would end up in the pattern). Also, you should go to Renoise preferences and disable any MIDI inputs that you are planning to manage using this tool - otherwise, incoming MIDI from those devices can still be recorded by accident. You can still continue using other controllers, of course.
Also, there is a limitation when a group track contains multiple assigned tracks: the next time you load the song those instruments might end up being assigned to the same track.
I'm thinking of various ways to solve this problem, but for now, simply create a group track for each assigned track (or let the tool do it for you).
The tool is serving a particular purpose. But it could be interesting to add more "instrument features" to the grid. For example, global control of transpose or volume. An instrument "mixer" of sorts