There are certain established global content identifiers such as [Book ISBNs](https://en.wikipedia.org/wiki/ISBN), [Podcast GUIDs](https://podcastnamespace.org/tag/guid), and [Movie ISANs](https://en.wikipedia.org/wiki/International_Standard_Audiovisual_Number) that are useful to reference in nostr events so that clients can query all the events assosiated with these ids.
`i` tags are used for referencing these external content ids, with `k` tags representing the external content id kind so that clients can query all the events for a specific kind.
Book ISBNs MUST be referenced _**without hyphens**_ as many book search APIs return the ISBNs without hyphens. Removing hypens from ISBNs is trivial, whereas adding the hyphens back in is non-trivial requiring a library.
Movie ISANs SHOULD be referenced _**without the version part**_ as the versions / edits of movies are not relevant. More info on ISAN parts here - https://support.isan.org/hc/en-us/articles/360002783131-Records-relations-and-hierarchies-in-the-ISAN-Registry
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### Optional URL Hints
Each `i` tag MAY have a url hint as the second argument to redirect people to a website if the client isn't opinionated about how to interpret the id:
