According to the GDPR laws, when are SAP customers considered data controllers?

According to the SAP S/4HANA Cloud, public edition Learning Journey1, according to the GDPR laws, SAP customers are considered data controllers when they extract legacy data in preparation for migration. GDPR is a regulation that protects the privacy and personal data of individuals in the European Union. Data controllers are entities that determine the purposes and means of processing personal data. Data processors are entities that process personal data on behalf of data controllers. SAP is a data processor for SAP S/4HANA Cloud customers, while customers are data controllers for their own legacy data.