Sub-projects are simply normal project folders that can be created within another existing project. Sub-projects allow a very large project to be broken down into more manageable pieces. For example, a large commercial construction project could be managed in one project folder containing numerous sub-project folders for managing individual trade projects.
Sub-projects can also contain other sub-projects any number of levels deep. Project workflow interlocks can help synchronize sub-project workflows with their parent project workflow.
Depending on your vault’s configuration, not all project definitions may be able to be created as sub-projects. Consult a system administrator for more information on the project definitions that be used to create sub-projects in your vault.