What is special category personal data under the GDPR?

The GDPR specifically calls out the following types of personal data as "special category" personal data: racial or ethnic origin, political opinions, religious or philosophical beliefs, or trade union membership, and the processing of genetic data, biometric data for the purpose of uniquely identifying a natural person, data concerning health or data concerning a natural person’s sex life or sexual orientation (GDPR Art 9)

A recent case found that special category data is not only such data, but also any personal data that when processed could reveal special category data (see link). This ruling substantially broadens the scope of personal data that is subject to increased restrictions, and will make processing of personal data more complicated.