Your doctor notes your diagnosis in the form of a code, called the ICD diagnosis code, on the Arbeitsunfähigkeits-Bescheinigung [statement of fitness for work] for the health insurance fund.
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Diseases, illnesses and health problems are defined on a standardised basis using this internationally recognised classification system. The abbreviation ICD stands for "International Statistical Classification of Diseases and Related Health Problems".
If you're interested: The TK also offers a free smartphone app to check on all ICD-10 diagnoses (only available in German language):