If you want to limit the activities/services that your employees can access while filling out their time card, you can use projects assignments. For example, if you want project managers to be able to choose from certain activities that apply to them and CAD designers to choose from other activities that apply to their position, you can set up project assignments to assign specific employees, activities or expense items to relevant projects. To do so, go to Projects and select the project you want to apply assignments to. Then, go to General > Assignment tab to add assignments. But remember that project assignments only apply to time and expense entry. Check CORE Help Center for details.
However, if you want to assign projects, activities, and expenses to employees throughout CORE, Manage Access is what you are looking for. This shortens the drop-down lists and make all data entry more accurate. However, you might want to assign the admin or managers to all relevant projects so as not to restrict any list or screen from them. Check CORE Help Center for details.
Please note that Manage Access is a security feature and if you apply both features in CORE, in case of a conflict, it takes precedence over project assignments.