A subproject is used to include another set of modules in your event, often via the menu. Often used scenarios are break outs or surveys. You can access the subproject from a tab in 'Live mode'. From here you can control the subprojects timeline just as your normal timeline.
2. How to
When in live mode, click on 'Subproject: name', and then run the sub project.
To go back to current project, click on 'Timeline'.