With the General Data Protection Regulation (GDPR) about to go into effect on May 25th, you should be aware of what Personally Identifiable Information (PII) could be stored in Sitecore 9 xConnect.
These fields are easily identifiable if you have installed Sitecore Data Exchange Framework 2.0+ and the xConnect Provider (provides the ability to read and write data to the collection and reference data services of Sitecore xConnect).