A structural specialization can incorporate elements from unrelated domains or other
structural specializations by referencing them in the content model of a specialized
element. The elements included in this manner must be specialized from ancestor content that
is valid in the new context. If the reusing and reused specializations share common
ancestry, the reused elements must be valid in the reusing context at every level they share
in common.
Although a well-designed structural specialization hierarchy with controlled use of domains
is still the primary means of sharing and reusing elements in DITA, the ability to also
share elements declared elsewhere in the hierarchy allows for situations where relevant
markup comes from multiple sources and would otherwise be developed redundantly.