CORE users often utilize project assignments or employee assignments. When you use employee assignment from the Manage Access screen in CORE (Settings > Access & Permissions), you assign specific projects, activities and expenses to employees. When you use project assignment, you assign specific employees, activities and expenses to a project.
The Assignment tab of a project profile is where of Team Members, Activities, and Expenses are assigned to a project record. When assigned, these members, activities, and expenses are the only ones available for use for that project. Note that this feature works only on time and expense entry modules. Check out this article on assigning teams and projects for details.