All audio and events that you generate in the Composer can be sent to Racks.
Racks are an easy and flexible way of combining plugins and modules, see the detailed info on Racks.

The Rack Desk shows all racks of a Composer.
Each rack can be shown in an expanded or collapsed version.
This can be controlled by the little button at the top left of each rack. This button also has a context menu with extra functions.

Each rack can receive audio and events from any clip 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, or by setting the output of a rack to another rack.
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 Rack A and Rack B to a common rack C.
Plug in a reverb in rack C and so that common rack C will process the audio from A and B thru its plugged in reverb.

Also see Using Insert And Send Effects.

Going into deeper detail:
In fact a rack desk is the collection of racks within a specific modular area.
So each MUX Modular can have its own rack desk.
As the composer also is a modular module itself, it can also have its rack desk and that's the one you see integrated in the composer editor.
 
So if you make a MUX that includes racks (eg. the Mudrum preset) you can add a rack desk for this MUX on the front panel of this MUX.
How: Edit Front Panel -> Add Component -> Rack Desk -> Now click on a modular area for which you want to add a rack desk.
Attention: It must be a modular area that is inside this MUX. It can be nested deeper, but it has to be a (sub)child to that MUX modular.

More Info

• At the top left of the racks in the rack desk, there is a little button that lets you collapse/expand the rack.
• Shift+clicking this collapse/expand button will collapse/expand all racks.
• Right-click this collapse/expand button for extra options, e.g. to control whether racks should automatically collapse/expand according to the focused module.
• To make relations between tracks and racks more visual, you can color racks by drag-dropping a color from the Color Picker.
• You can popup the rack desk context menu by right-clicking the background of the mixing desk itself, thus at the very right of the mixing desk.
• You can add a new rack via the [+] button at the right of the right-most rack and via rack desk's context menu -> "Add New Rack".
• You can drag-drop any existing module on that [+] button, which will create a new rack with that module moved/copied into slot 1. Hold Ctrl to copy.
• You can drag-drop rack and MUX presets and VST / CLAP plugins on that [+] button, which will create a new rack with that preset/plugin loaded into slot 1.
• Note that scrolling using the mouse-wheel is explicitly disabled for the vertical zone where the volume faders are. The reason for that is that if you would be scrolling and you would accidently hover a volume fader you would unintendedly change your mix. Disabling the mouse-wheel scrolling in the volume fader zone prevents that.
• The rack desk can be used in docked mode or in windowed mode. You can toggle between these modes via the mixing desk context menu -> "Switch To Docked/Windowed Mode".