- What products are in inventory?
- What users have accounts on the system?
- What tests have been run?
But many domains also include information about the provenance of data, e.g.
- Who said this data was correct?
- When should this data be published?
- Where did this data come from?
- Why was this correction necessary?
How should you model this "data about data"? Datomic stores (and lets you store) facts about transactions themselves. In other words, transactions are themselves reified as data in the system.
Tim Ewald gave a great talk about this at Datomic Conf. His talk and all the other Datomic Conf talks are now freely available online.