This purely comes down to exactly how you are storing, processing, or transmitting cardholder data. By default, you would need to complete SAQ D, which includes the full set of PCI DSS requirements. However, many merchants store, process or transmit cards via specific methods (for example only using a third party to take payments, or only taking payments only via P2PE payment terminals). For these situations, you can potentially use one of the alternative SAQs, which only list the relevant requirements. These SAQs include SAQ A, SAQ A-EP, SAQ B, SAQ B-IP, SAQ C-VT, SAQ C, SAQ P2PE, SAQ D. Service providers would always have to complete SAQ D for Service providers.