@keys in

A reference to a key is *never* a reference to the topicref that defines the
key but a reference to the resource *bound to* the topicref that defines the
key.

That is, a reference to a key name is *always* an indirection through the
key-defining topicref. DITA 1.2 provides no way to say "this key reference
is addressing the topicref with this key instead of the resource bound to
the topicref with this key".

A key may be directly bound to the following types of resources:

1. To a topic (the topic directly or indirectly addressed by the
key-defining topicref).

2. To a map (the map directly or indirectly addressed by the key-defining
topicref)

3. To a non-DITA resource (e.g, a graphic directly or indirectly addressed
by the key-defining topicref)

4. To a topicref in the same or a different map, using an URL with a
fragment ID that is the ID value the topicref within the target map, e.g.: