explain semantics of format object better pace #413

spec/spec.md

@@ -305,15 +305,21 @@ be ignored, unless otherwise specified by a [[ref:Feature]];
   are being used for.
 - The [[ref:Presentation Definition]] ****MAY**** include a `format` property.
   Some envelope transport protocols may include the value of this property in
-  other locations and use different property names (See the [Format Embed Locations](#)
-  section for details), but regardless of whether it resides at the default
-  location (the `format` property of the `presentation_definition` object), the
-  value ****MUST**** be an object with one or more properties matching the registered
-  [Claim Format Designations](#claim-format-designations) (e.g., `jwt`, `jwt_vc`,
-  `jwt_vp`, etc.). The properties inform the [[ref:Holder]] of the [[ref:Claim]]
-  format configurations the [[ref:Verifier]] can process. The value for each
-  claim format property ****MUST**** be an object composed as
-  follows:
+  other locations and use different property names (See the [Format Embed
+  Locations](#) section for details), but regardless of whether it resides at
+  the default location (the `format` property of the `presentation_definition`
+  object), the value ****MUST**** be an object consisting of keyed arrays.  Each
+  array should be keyed to a [Claim Format
+  Designation](#claim-format-designations) (e.g., `jwt`, `jwt_vc`, `jwt_vp`,
+  etc.) registered in the governing
+  [registry](https://identity.foundation/claim-format-registry/#registry). The
+  values in each array should only be values of `alg` and/or `proofType` that
+  are valid according to this registry (and the upstream registries per
+  claim-format linked from it). These arrays of strings thus function as
+  enumerations of supported profiles for each claim format, informing the
+  [[ref:Holder]] of which [[ref:Claim]] format configurations the
+  [[ref:Verifier]] can process. The value for each claim format property
+  ****MUST**** be an object composed as follows:
     - The object ****MUST**** include a format-specific property (i.e., `alg`,
       `proof_type`) that expresses which algorithms the [[ref:Verifier]]
       supports for the format. Its value ****MUST**** be an array of one or more