A design entity that is the root of a design hierarchy. To compile or simulate a design, you compile or simulate the top-level design entity.
A top-level design entity can contain any number of lower-level design entities (or, subdesigns), and is the parent for the lower-level design entities.
When you create a project, you specify a top-level design entity for the project. You can also specify a different top-level design entity for each of a project's revisions.