Field Audit Trail and Platform Encryption are two different Salesforce security tools that can be used to meet the requirement of storing protected health information (PHI). Field Audit Trail allows government agencies to track changes to sensitive data fields over time and set retention policies for field history data. Platform Encryption allows government agencies to encrypt sensitive data at rest while preserving critical platform functionality. Reference: https://trailhead.salesforce.com/content/learn/modules/public-sector-solutions-design/secure-data-with-platform-encryption