Personal data is any information that can be used to identify a person. Quotes, photos, video and audio recordings may also be personal data. This also applies even though the individual information must be linked to other information before it is possible to identify the person. Personal data is only anonymised when it is impossible to identify people based on the information.
A quote can also constitute personal data: ‘I am the Queen of Denmark’. This quote does not contain a name, but nevertheless specific information about a special office. The data may apply to different people, but in conjunction with a date, it will be possible to identify the person.
Thus, it is not enough to use a coding system in which a civil registration number is replaced by a code. As long as it is possible to backtrack the information using the code, the process is called pseudonymisation, and the information is still personal data. This applies even if only one person has the code for the information. You therefore need to be very aware of whether your information is personally identifiable and whether you have mastered the relevant security measures required by the GDPR. It is not certain that a piece of information is anonymous in a legal sense, even if you believe it is anonymised.
You can find examples of personal data here.