Sub projects are a child project that belong to a parent or main project that you are working on. This allows you to have multiple stages linked up to one project. For example you are building an apartment block, you can have Apartment Block Project as the parent project and then have each level as a sub project.
Adding A Sub Project
Click on Add New Project in the Projects List View
Select the Client
Enter a Project Name
Select a Parent Project
Enter an Address
Choose any of the project rules
Click Save and Close
Sub Project Details
After saving the project, you have all the same functionality of an existing project, which you can read about here
You can see in the project details screen, the it show you the parent project it is linked to. After you've linked the sub project to a parent project, it cannot be changed.
NOTE: When you archive a parent project, all the sub projects linked to it will be archived too.