Clarify that subject references a separate DAG

Signed-off-by: Brandon Mitchell <git@bmitch.net>

image-index.md

@@ -93,7 +93,7 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 - **`subject`** *[descriptor](descriptor.md)*
 
     This OPTIONAL property specifies a [descriptor](descriptor.md) of another manifest.
-    This value, used by the [`referrers` API][referrers-api], indicates a relationship to the specified manifest.
+    This value defines a loose relationship to a separate DAG structure, and is used by the [`referrers` API][referrers-api] to this manifest in the list of responses for the subject digest.
 
 - **`annotations`** *string-string map*
 

manifest.md

@@ -94,7 +94,7 @@ Unlike the [image index](image-index.md), which contains information about a set
 - **`subject`** *[descriptor](descriptor.md)*
 
   This OPTIONAL property specifies a [descriptor](descriptor.md) of another manifest.
-  This value, used by the [`referrers` API](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers), indicates a relationship to the specified manifest.
+  This value defines a loose relationship to a separate DAG structure, and is used by the [`referrers` API][referrers-api] to include this manifest in the list of responses for the subject digest.
 
 - **`annotations`** *string-string map*
 
@@ -257,6 +257,7 @@ The decision tree below and the associated examples MAY be used to design new ar
 
 _Implementers note:_ artifacts have historically been created without an `artifactType` field, and tooling to work with artifacts should fallback to the `config.mediaType` value.
 
-[iana]:         https://www.iana.org/assignments/media-types/media-types.xhtml
-[rfc6838]:      https://tools.ietf.org/html/rfc6838
-[rfc6838-s4.2]: https://tools.ietf.org/html/rfc6838#section-4.2
+[iana]:          https://www.iana.org/assignments/media-types/media-types.xhtml
+[referrers-api]: https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers
+[rfc6838]:       https://tools.ietf.org/html/rfc6838
+[rfc6838-s4.2]:  https://tools.ietf.org/html/rfc6838#section-4.2