Cross-deliverable addressing and linking
When maps are referenced and the value of the @scope attribute is set to "peer", the implications are that the two maps are managed in tandem, and that the author of the referencing map might have access to the referenced map. Adding a key scope to the reference indicates that the peer map should be treated as a separate deliverable for the purposes of linking.
The keys that are defined by the peer map belong to any key scopes that are declared on the <topicref> element that references that map. Such keys can be referenced from content in the referencing map by using scope-qualified key names. However, processors handle references to keys that are defined in peer maps differently from how they handle references to keys that are defined in submaps.
DITA processors are not required to resolve key references to peer maps. However, if all resources are available in the same processing or management context, processors have the potential to resolve key references to peer maps. There might be performance, scale, and user interface challenges in implementing such systems, but the ability to resolve any given reference is ensured when the source files are physically accessible.
Note the inverse implication; if the peer map is not available, then it is impossible to resolve the key reference. Processors that resolve key references to peer maps should provide appropriate messages when a reference to a peer map cannot be resolved. Depending on how DITA resources are authored, managed, and processed, references to peer maps might not be resolvable at certain points in the content life cycle.
The peer map might specify @keyscope on its root element. In that case, the @keyscope on the peer map is ignored for the purpose of resolving scoped key references from the referencing map. This avoids the need for processors to have access to the peer map in order to determine whether a given key definition comes from the peer map.
Example: A root map that declares a peer map
Consider the DITA maps map-a.ditamap and
map-b.ditamap. Map A designates Map B as a peer map by
using the following markup:
<map>
<title>Map A</title>
<topicref
scope="peer"
format="ditamap"
keyscope="map-b"
href="../map-b/map-b.ditamap"
processing-role="resource-only"
/>
<!-- ... -->
</map>
In this example, map-b.ditamap is not a submap of Map A; it is a
peer map.
Example: Key resolution in a peer map that contains a @keyscope attribute on the root element
Consider the map reference in map Map A:
<mapref keyscope="scope-b" scope="peer" href="map-b.ditamap" />
where map-b.ditamap contains the following markup:
<map keyscope="product-x"> <!-- ... --> </map>
From the context of Map A, key references of the form "scope-b.somekey" are resolved to keys that are defined in the global scope of map B, but key references of the form "product-x.somekey" are not. The presence of a @keyscope attribute on the <map> element in Map B has no effect. A key reference to the scope "scope-b.somekey" is equivalent to the unscoped reference "somekey" when processed in the context of Map B as the root map. In both cases, the presence of @keyscope on the root element of Map B has no effect; in the first case it is explicitly ignored, and in the second case the key reference is within the scope "product-x" and so does not need to be scope qualified.