Meta-schema URI

The URI a schema's own $schema key holds, which declares the schema's dialect.

It is a different use of the same key a document carries, found inside a schema rather than a document. There, $schema is the meta-schema URI that declares the schema’s dialect, for example https://json-schema.org/draft/2020-12/schema. manni meta reads it to decide which validator to compile the schema with. Don’t confuse it with the document $schema key: one selects a dialect, the other selects which schema validates a document.