This is a system module to connect another project. For example, if you want to connect a survey or if you have multiple break out sessions (all projects can still be run with their own screens/control panels even if connected to other projects).
In this resource:
To add a subproject module, go to the project timeline and 'Add module', choose 'Subproject', name it, and press 'select'. The new module will appear in the modules library.
You can also add a subproject directly on the timeline by clicking the plus sign on the timeline.
Click on your new sub project module to select what project you want to connect to it.
Drag and drop your subproject module to the slot you want it in the timeline or in the menu.