A merchant wants to store past purchase history in Marketing Cloud to use for segmentation...

Field Level Encryption is a feature that allows encrypting data at field level in Marketing Cloud. It encrypts data at rest to facilitate compliance with data privacy and security regulations. It also allows importing encrypted data into Marketing Cloud and decrypting it at send time. References: https://help.salesforce.com/s/articleView?id=sf.mc_overview_field_level_encryption.htm &language=en_US&type=5