On this element, the global xml:id attribute must be supplied to specify an identifier for the point at which this element occurs within a document. The value used may be chosen freely provided that it is unique within the document and is a syntactically valid name. There is no requirement for values containing numbers to be in sequence.
Example
<s>The anchor is he<anchor xml:id="A234"/>re somewhere.</s> <s>Help me find it.<ptr target="#A234"/></s>