docs: rewriting OCM OCI repository mapping

doc/03-persistence/02-mappings.md

@@ -2,19 +2,24 @@
 
 This chapter describes how OCM model elements are mapped to elements of a persistence layer.
 
-OCM model elements are mapped to various storage technologies. The interoperability layer for a client tool is typically the API of the storage backend. This avoids the need for providing an OCM server infrastructure.
+OCM model elements are stored using various backend technologies. The interoperability layer for a client tool is typically the API of the chosen storage backend, avoiding the need for a dedicated OCM server.
 
-An implementation of this layer MUST implement this mapping by supporting the [mandatory abstract model operations](./01-operations.md#mandatory-operations). It SHOULD implement the [optional operations](./01-operations.md#optional-operations) too.
+An implementation of this layer **MUST** support the [mandatory abstract model operations](./01-operations.md#mandatory-operations).  
+It **SHOULD** support the [optional operations](./01-operations.md#optional-operations).
 
 ## Storage Backend Mappings for the Open Component Model
 
-The OCM specification describes an interpretation layer on-top of well-known storage backend technologies used to store OCM component versions.
-
-Therefore, for every storage technology a mapping must be defined to ensure interoperability between different OCM implementations.
+The OCM specification provides an interpretation layer on top of storage technologies used to persist component versions.
 
+For every supported technology, a mapping **MUST** be defined to ensure consistent interoperability across implementations.  
 These mappings describe:
 
-- the repository specification type and format used to specify a dedicated repository instance
-- the mapping of the OCM elements to the elements provided by the storage technology.
+- the repository specification type and format for identifying a repository instance, and
+- how OCM model elements are mapped to the artifacts or structures provided by that backend.
+
+These mappings are defined as part of the [extensions](../04-extensions/03-storage-backends/README.md).
+
+## Note on OCI Storage Backends
 
-The mappings are defined as part of the [extensions](../04-extensions/03-storage-backends/README.md)
+The OCI storage backend mapping now supports **both manifest-based and index-based representations** of component versions.  
+The detailed rules and normative behavior of the index-based format are defined in the corresponding OCI backend extension.