The I-766 is an Employment Authorization Document (EAD). The EAD Category Definition provides the categories of aliens or classes of admission (COA) that fall under the EAD category.
Note that Not All Visa Types Issue an EAD. Some Visa Types like an HB-1 has the “right to work” built into the Visa type. You will not be able to obtain an actual EAD card for those clients because the USCIS will not issue a card when the Visa Type has the “right to work” included.