Fuente: Wipo "digitalization" A system for tracking usage of digital content on user devices. Content sites for distributing digital content over a computer readable medium to users. The content sites associate unique content identifier with the content associated. Electronic stores coupled to a network sell licenses to play digital content data to users. The licenses contain a unique transaction identifier for uniquely identifying the transaction, and the licenses contain a unique item identifier for uniquely identifying at least one item in the transaction. Content players, which receive from the network the licensed content data, are used to play the licensed content data. The content players produce a purchase identifier based upon the mathematical combination of the content identifier, the transaction identifier and the item identifier.