Dear abhishek, your BC-D certificate will mention all the necessary details like the sub category of your Category. This states that you belong to a certain Caste which falls under BC-D of OBC category. Because this is issued in a certain pre approved format, you dont need another certificate stating that you are from OBC. In general BC-D is the sub category of OBC and no other category. So once it is mentioned BC-D, it is understood that you belong to BC-D of OBC.