The ATC classification (Anatomical Therapeutic Chemical Classification System) is an official classification for pharmacological substances.
Active substances are divided into different groups according to the organ or organ system they affect and according to their anatomical, therapeutic and chemical properties.

The active substances are assigned a defined daily dose (DDD). 
This DDD is the assumed average daily maintenance dose for the main indication of an active substance in adults.

Reference:

ATC/DDD Index 2026