Metadata may be stored with items, entities, and relationships between items/entities. It takes the form of a simple key-value store and is encoded as a JSON object in the database. This unstructured format is simple and highly flexible.

Metadata keys+values should be represented consistently across data sources for those which provide equivalent metadata. This forms metadata conventions that the frontend UI can more or less depend on. These conventions are documented on this page. For example, any data source providing velocity information for an item should use the key "Velocity" with an integer value in meters per second.

The frontend can be programmed to use these metadata conventions in various contexts to provide a richer experience. For example, metadata is used to show where entities are in photos. Being documented on this page does not mean that the UI necessarily uses the convention, but it may do so in the future without difficulty/ambiguity.

Keys take a human-natural form with spaces and the first word, proper names, and initialisms are capitalized.

These metadata conventions are not an external API contract, so they are subject to change. (But we still prefer not to change them.)