All audio and events that you generate in the Composer can be routed to Racks.
A Rack is a collection of plugins that process your audio and events and route it to an output.
A Rack also has an audio gain, pan and mute control that processes the audio just before the first post-fader plugin. (more on this below)
Racks are a very powerful and flexible!
For example, you can set up a Rack "Bass" to have a nice bass synth plugged in, and also a subtle audio chorus plugin that furtherly processes the bass sound. The Rack is set to output to the main audio output.
Now you can set the Target of a bass sequence part in your composition to be Rack "Bass", and so all events of that sequence go to the "Bass" Rack, where the synth will make sound of it, and the chorus will fatten that sound 
It's important to understand that a Rack can receive audio as well as events. It depends very much on what you plug in whether this audio and events are furtherly processed.
In the above example, sending audio to Rack "Bass" won't have much effect because generally a synth plugin does not process audio input, but only event input, which it then turns into audio, i.e. the bass sounds.
At the other hand, you could setup another rack that receives events from a sequence part, with a MIDI effect plugged in (e.g. an arpegiator), and then sending the result to a hardware MIDI port so to send the resulting events to one of your external synths. Now in this case, it won't make much sense to send audio to this rack, because a MIDI effect, nor the MIDI port can do anything with the audio.
So it all depends on what you input into a Rack and which plugins are in the Rack. LUNA does not set any limits here, and thus you are free to make any combinations of plugins and routings of audio and event signals.
Sometimes, you might not hear what you first would expect, because audio or event signals don't come thru due to certain plugins. But if you then precisely analyse how signals flow, you'll find the reason.
The good thing about this is creative freedom!
There are so many plugins out there, some of them are pure for audio, some other are pure for events, and there also exists plugins that process both audio an events, or convert audio into events or vice versa.
LUNA's Racks let you PLAY with it
The Rack Window shows all used Racks.
The focussed Rack is shown in full width, the others are shown as smaller strips, so to have a better overview. You can focus on a Rack by clicking its background. Then it widens so to show more detail.
Each Rack can receive audio and events from any Part in the Composer, but can also receive audio and events from other Racks! This is done by plugging in a "Send" from one Rack to another.
For example: if you want to apply the same reverb on synth A (in Rack A) and synth B (in Rack B), then send the audio from the Rack A and Rack B to a common Rack; than that common Rack will process the audio from A and B thru its plugged in reverb.
Note that when a Rack only receives only mono audio, then its output will be mono too. But as soon as it also receives a stereo signal, then the output will be stereo, and any received mono signals are automatically turned into stereo too.
All Racks work in the same way:
- At the top there is a label that you can edit.
- At the right side, there are multiple Plug Slots, in which you can plug in a synth or effect, or plug in a Send (cfr the example above).
- At the left of these Plug Slots, there is a Pre/Post Indicator, which you can move up or down by clicking at the left of the slots.
The name Pre/Post comes from the Pre-Fader/Post-Fader analogy.
The input signal (audio and/or events) is directly processed by all plugins before the POST. Then the Volume/Pan/Mute is applied to the audio, events just pass thru. Then the signal(s) stream(s) thru the first Post plugin, and thru the following ones.
- You can double-click a plug slot to open the plug editor.
- At the right of a Send is a little knob for controlling the send gain, i.e. for controlling how much is sent.
- You can rearrange the order of the plug slots: Use shift+click and drag the slot up or down.
- Right-clicking a volume slider resets it to 0 dB.
- Right-clicking the background pops up a context-sensitive menu with more options.
- Right-clicking the output field lets you choose which type of output must be displayed: Audio or Event. Note that they do exist at the same time (not necessarily effectively outputting), but here you choose which one is displayed.
- You can mute a Rack via its mute button. Right-clicking the mute button pops up a context menu with Solo and Unsolo options. Note that the mute is applied just before the first POST plug. This means that e.g. any sends before this point are still sending signal.
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