Must all resources have JSON:API representations?

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As far as I understand the JSON:API specification the relationship object must point to a JSON:API resources.

Relationships

The value of the relationships key MUST be an object (a “relationships object”). Members of the relationships object (“relationships”) represent references from the resource object in which it’s defined to other resource objects.

JSON:API — Latest Specification (v1.1)

For use cases like the once described by you I usually use an attribute, which value is a URL to fetch the data. E.g. an user resource may have an avatar attribute, which is an URL pointing to an image. Similar an attachment resource could have a content attribute, which is an URL to download the raw content of that attachment.

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