Using JSON Pointer fragments requires knowledge of the structure of the schema. When writing schema documents with the intention to provide re-usable schemas, it may be preferable to use a plain name fragment that is not tied to any particular structural location. This allows a subschema to be relocated without requiring JSON Pointer references to be updated.

The "$anchor" and "$dynamicAnchor" keywords are used to specify such fragments. They are identifier keywords that can only be used to create plain name fragments, rather than absolute URIs as seen with "$id".

The base URI to which the resulting fragment is appended is the canonical URI of the schema resource containing the "$anchor" or "$dynamicAnchor" in question. As discussed in the previous section, this is either the nearest "$id" in the same or parent schema object, or the base URI for the document as determined according to RFC 3986.

Separately from the usual usage of URIs, "$dynamicAnchor" indicates that the fragment is an extension point when used with the "$dynamicRef" keyword. This low-level, advanced feature makes it easier to extend recursive schemas such as the meta-schemas, without imposing any particular semantics on that extension. See the section on "$dynamicRef" (Section 8.2.3.2) for details.

In most cases, the normal fragment behavior both suffices and is more intuitive. Therefore it is RECOMMENDED that "$anchor" be used to create plain name fragments unless there is a clear need for "$dynamicAnchor".

If present, the value of this keyword MUST be a string and MUST start with a letter ([A-Za-z]) or underscore ("_"), followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), and periods ("."). This matches the US-ASCII part of XML's NCName production [xml-names]. 

Note that the anchor string does not include the "#" character, as it is not a URI-reference. An "$anchor": "foo" becomes the fragment "#foo" when used in a URI. See below for full examples. 

The effect of specifying the same fragment name multiple times within the same resource, using any combination of "$anchor" and/or "$dynamicAnchor", is undefined. Implementations MAY raise an error if such usage is detected.