If you want to limit the activities/services that your employees can access while filling out their time card, you can use the Projects Assignments feature. For example, if you like 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 will set up project assignments to assign specific employees, activities or expense items to relevant projects. But remember that project assignments only apply to time and expense entry. Check the Core Help for details.
However, if you want to assign projects, activities and expenses to employees and that too throughout Core, Employee Control is what you are looking for. This will shorted 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 the Core Help for details.
Please note that Employee Control is a security feature and if you apply both features in Core, in case of a conflict, it takes precedence over Project Assignments.